It has long been a problem for sequels of games with unofficial patches, that the sequel is often indeed a marked improvement over the predecessor, yet a big step back from the predecessor-with-a-decade-of-community-improvements.

One of the latest, most lamented cases of such a sequel is Mount & Blade Bannerlord, but many others exist: Crusader Kings 3, Victoria 3 are just two more "honourable mentions".

Keeping this in mind: How does the current state of JA3 shape up against the current state of its predecessor?

Is it (already) worth it?

Going to be honoust here.
As you said the current state of JA2 v1.13 is with official patches and almost two decades community improvements and modding, thus you cannot fairly compare the latest build of JA2 v1.13. (For those not aware, 1.13 is just a label, thousands of builds versions have been made with changes and all)
If you want to compare it you will have to compare it to the JA2 base version.

I do have to say that JA3 looks very potential, sure there are issue's like AI could be smarter and some mechanics seem weird. It did however try to keep things as much as possible to the official JA feeling in the way most modern tactical design in. In the sense of the XCOM: Enemy Unknown positioning/cover and such. And it allows new people into the genre, and lets be honoust in this regard, JA2 original was a good entry for new people but had issue's that was off putting back that day, and those willing to get into it now get the golden version with v1.13 and that has a lot of moments of difficulty that put people off.

Aside from AI though, the issue's like the UI feels weird in non-combat when prepping and some combat elements it does feel proper attempt. And with proper investment from developers and additions it could grow rather big.
Modding support seems to be there, though didn't check how. I am worried it might be the steam monopoly only variant. And if that is the case then it will only bigger on steam. (In my opinion EU should force steams mods not to be only steam only imho, its heavy monopolistic addition)

I like it, but I do think it is best to watch a video or stream of those fan of JA2 and how they are experiencing it. Do not watch a live one though unless its a fresh starting one. You get a better opinion on that as it is easier to see the quirks then explain it and if you are willing to accept those or wait till adjusted.


Addition:
I really do hope sales go well outside steam, and that they will add more mercs, maybe even M.E.R.C. again :P (they were cheap and horrible, but very good depending on how done)
The backpack and heavy customization is gone, depending on preference this is nice or not. It does remove a lot of weird clutter management and is consolidated into strength on how much. Some might say it removes realism, but to be fair the way JA2 did was also not realistic.

I've played JA3 for 6 hours so far.

I can confidently say it feels like the strongest successor to JA2 yet, after so many lukewarm at best, botched at worst attempts.

I haven't really played vanilla JA2. Compared to 1.13, it's more streamlined, but a lot of that is a collection of smart moves.

* The arsenal and attachments are less varied, but more meaningfully different. A lot of the 1.13 variety is admittedly stat porn.
* Maps are smaller but tend to have more personality: less empty jungle to trudge through. Still, we're talking about ~100 handcrafted tactical maps.
* It's intended to be very moddable from the get-go, which may well be a godsend for its expansion and overall longevity.
* The mercs feel quite JA-like, and while I found the outfits initially jarring, they grew on me. Especially understanding so far merc appearance had been but a scarcely animated mugshot and a color combination of pants, shirt and an A/B/C body type.
* Bobby Ray's so far appears to be absent, but enemies regularly drop loot, and you can procure some old rifles and a couple of dingy AKs within 2-3 battles. No extended suffering with pistols vs. rifle-armed goons.
* No hints of land or ground (motorized) transportation so far, but given the land's riverine disposition, I get the impression much of the faster travel is going to be via boat, originating from port towns.
* Potentially further undiscovered stuff!

While generally a well-made game, I miss a lot of features from JA2/1.113, as well, of which I'm unsure we'll be seeing again in JA3.

My complaints so far:
-Barely-existent economy.
-No real shops, online or otherwise.
-No pausing while in real-time.
-Extremely streamlined """crafting economy""" that is supposed to make up for lack of economy, bartering and suppling your mercs with weapons, ammunition and gear, yet fails to satisfactorily achieve so.
-Extremely sparse loot, from combat and exploration both. The ammo shortage is severe.
-Linear gear and weapon upgrade paths.
-Extremely simplified inventory. All mercs have the same loadout inventory, and additional backpack size is solely determined by individual merc strength.
-Squad size limited to 6 mercs.
-Finicky looting/action UI, where actions(skill checks) will be performed by the closest merc instead of the most proficient merc.

Just putting back Bobby Ray's, or another well-stocked shop, would go far to alleviate a lot of my issues with the same.
Making weapons side-grades, instead of straight-up upgrades, would help too, both with enjoyment with the game, and also with the balance(once you manage to set up a reliably source of ammunition).

+1

JA3 is a game, but it`s not JA.

Other developers can do it too, can't they? Example: Xenonauts 2, which is out now on Tuesday and has been bought!

The developers stand by their design! Old, proven and loved by many! You don't have to move with the times and dear JA3 developers, you didn't have to reinvent the wheel!

I ask for many updates, the game desperately needs them.

Xenonauts stand by their design? They change a lot of things if actually compare them and the Xenonauts 2 is actually more what backers paid for to have in Xenonauts, in their own discords they even said Xenonauts 2 is what 1 actually had to be but did not become because kep stubborn on the strange abandoned engine. You cannot compare these two.

Also to negative points, that is how it is felt but do want to point out a few things:

Things that are valid:
-No pausing while in real-time.
Valid but only missing for pre-planning and attack and to see where you gain shelter before combat

-Finicky looting/action UI, where actions(skill checks) will be performed by the closest merc instead of the most proficient merc.
Valid annoying thing yes, though other JA also did that funny enough. Community patches fixed that after hundreds of changes after it, so for a long time that was also the case in JA2

Things that you might be used to in other JA due to modding thus not fair:
-Barely-existent economy.
All JA had no real economy if you can even call it that. Current way is almost same as others, anything different you experienced than is from mod changes or strange added settings by community. Just get mines and that is it.

-No real shops, online or otherwise.
There are shops, but no Bobby Ray. It was to elevate an issue pointed out later and a way to get gear during progressing but if you didn't manage to get from enemies or bother fixing them. Shops in other JA were useless, mods changed them a bit so not fair comparison.
Also note that JA2 1.13 depending on settings can make it insane overpowered or lacking, where the lacking from community is rather the way to keep it "balanced"

-Extremely streamlined """crafting economy""" that is supposed to make up for lack of economy, bartering and suppling your mercs with weapons, ammunition and gear, yet fails to satisfactorily achieve so
Exact same as other JA, Bobby Ray was only way to get the best, the rest was getting from enemies if you were lucky and fixing them. Most times you were running with crap that you couldn't repair till Bobby had better gear and had a peddler with helicopter.

-Extremely sparse loot, from combat and exploration both. The ammo shortage is severe.
Exact same as other JA unless you played on easier modes or the 1.13 plenty settings, the recommended ones are also shortage. Community patch made it actually sadly plentier in some cases on defaults. In JA3 you have to make ammunition instead of buying it through operations. Actually forcing you to decide who to do that instead of getting a cheap M.E.R.C. to bring back and forth with helicopter. The lack of buying weapons like "Oh give me 20 bazooka's and 16 top of the line weaponry" at the snap of a finger adds in a challenge, though can guess it can be a nuisance.

-Linear gear and weapon upgrade paths.
Again the hundreds of changes added by 1.13 builds and community mods are actually flavor and most are shit, really shit. And depending on the mods some are absurd pumped due the national pov. Vinalla didn't have a lot of divergence, sure a bit more but a lot less than you think.

-Extremely simplified inventory. All mercs have the same loadout inventory, and additional backpack size is solely determined by individual merc strength.
Depends on taste, though do want to say that didn't have a lot of space for things in Vanilla or 1.13 either and was depending on outfit. Which was limited by your strength. Instead of both the reduction to strength only makes easier. A lot of streamers and video's out there of people constant watching non-stop bobby ray for that outfits to increase what they can carry and to be dumbfounded they are slow to the amount.

-Squad size limited to 6 mercs.
Depending on the JA game similar or less. JA1 had 8, JA2 had 6 orginally but community patch increased it and not because of wanting a bigger team but because the enemy could have 20+ with unfinished business and their harder mode thus needed some additionals. (And needs to be configured)

Unless playing a lot JA2 with mods most things pointed out were how you thought it was, was changed with community patch or mods or wished for changes. Now not saying they are invalid points, but they are more points for additions that could add flavour on how the game is played and for some. And hopefully they will add some, but others need to be added by the community.
Also again, JA3 while made as proper follow up of JA2, and the developers loving the 1.13 community patch, it is more a vanilla follow up than the v1.13. (And again the 1.13 patch has been thousand of times updated in almost 20 years, is you could see it more as v1.10992 and that is based only from Github 3992 changes commited where before Github they used a SVN where the build number was 7000 or so)

I've somehow ended up in the middle of ~20 enemies and is expected to somehow fight my way out through this.
There was no foreshadowing that this scenario would occur.
But, okay, it is theoretically feasible.
Except... I'm absolutely fucked ammo-wise.
Turns out my sniper rifle(M24), two of my assault rifles(Galil), my rifle(Gewehr) and my LMG(MG42) all use the same ammunition pool.
So after a couple of bursts from the LMG, 4 out of my 6 mercs are out of ammunition(7.62mm NATO) for their main weapon.
Which leaves me with two mercs carrying shotguns and a backup PDW.
I have 120 rounds of 7.62mm WP Match, 105 rounds of 7.62mm WP HP, 89 rounds of 7.62 WP Tracer, 37 rounds of 7.62mm WP Standard, 35 rounds of 7.62mm WP AP... But no. My guns will use 7.62mm NATO only!
This is bad game design.
There's no way for me to resupply ammunition for these guns. My only crafting options are WP rounds.
I didn't realise this game was Wasteland 2, where you absolutely cannot have two characters use the same ammo type or you'll run into shortages.

Actual shops could have remedied this. Bobby Rays could have remedied this. Better crafting could have remedied this.
A working economy, a reason to keep spare weapons around, and the capability and space to do so, would have remedied this.
Realistic loot would have remedied this. Especially after killing 20-or-so enemies in a scripted defence mission and getting practically jack shit as a reward for it.
(As far as I remember, the only ammunition I recovered after that fight were 41 rounds of 50BMG.)

There's a reason one of the most requested fixes to this game is a way to acquire more ammunition, whether that be a new Bobby Ray, a better shop implementation or a way to actually loot the enemies that you drop.

And lastly, in case you missed it, the topic specifically asks how this game measures up against a modded version of JA2, so that's what I'm going to compare it against.

I can understand the frustration on the ammo part and yes that can be annoying for people, and sorry to hear you got only guns that accept 7.62mm NATO bullets and than can happen RNG, and perhaps a setting to increase drops could help or a mod.
But do keep in mind that Bobby Ray had in originals limited amount of ammunition as well and the games, as this one, were designed with every bullet you shot had to feel as if would count and if not you choose if it is tactical reasonable to keep going or adjust.

And that is where a lot of points from ammo issue also come from, checked a lot of streams and most go full fighting and unwilling to move away or change approach to conserve. (Also a reason why Barry is considered a hot item to have due his deadly grenades renewal).
I think there is now a mod that makes it so that it should force 100% drops of after a fight

Heck even the most shout is actually showing percentage of hit which they themself provided a mod for. (Maybe tweakable to less, don't know) You can, and you can do it, and as can be pointed out that the comparison to it a bit skewed as it feels comparing two people of karate with one wearing an orange belt to one with a red belt. And why some things are there a bit in JA2 and why how they are, and if want pure comparison than also go for the actually meant settings of JA2 V1.13 (hard and very limited drops forcing a lot of pistol fighting and getting close with Steroids/Grizzly ;) )
If they invest and make the more mature as well as modding matures to it it will feel fair to compare JA2 v1.13 to it, and I freaking hope they invest into the game to handle the feedback to make it more configurable so issue's like the ammunition due to desired way to play can be handled as other things, and expand.

I can agree that the ammo situation could use some tweaking and balancing.

That said, is it really the game's fault that you didn't base your weaponry on your ammo inventory, and instead went for a specific type which is harder to supply?

Again, the game just launched and some polishing wouldn't go amiss, but it's also part of the Jagged Alliance theme to adapt to situations and work with what you have. It's not quite the kind of game which enables any arbitrary course of action just because.

It could even be that this is all by design, that since Western weapons are statistically better in-game than the Soviet stuff you find early on, given the rarity of their ammo you're supposed to use them more judiciously.

That's the fault of the linear weapon progression and overly simplified inventory system.
If I find a new gun that is in all aspects, from damage, range, accuracy, etc, better than what I've already got, why shouldn't I upgrade?
Even if I wanted to keep some spare weapons around, the overly-restrictive inventory system didn't allow me to do so.
Why can I order mercenaries from overseas, from all around the world, but I can't order a friggin' box of ammunition?
Why do all these guns use the same round, but deal wildly different amount of damage?
Why, if I'm supposed to rely on crafting ammunition, aren't crafting ingredients more readily available?

It's just bad game design, and nowhere near able to measure up against JA2 1.13.

I think they did a very good job of not releasing a second JA2
I'm greatly enjoying this game but then again I'm not using your tactics.
Ammo is plenty and some crafting materials are scarce which is a good thing because it keeps me excited about drops.

Yeah, well. JA2 still has an active community, 24 years after its release.
Let's see if JA3 even has half of that 5 years from now.

I already finished JA2 serveral times with and without mods. Why buy it again?

I still prefer JA2, it was bigger and the possibilities to interact with the civilians were more enhanced.
The strategical aspect was more important in that game.

But JA3 got it right, the combat is just fine and it has the better story telling.

Compared to 1.13 it lacks quite some stuff
but that is a unfair comparison in the first place if you want to know if JA 3 deserved the
name Jagged Alliance, well then compare it to Vanilla Jagged Alliance 2

and i do say, yes it definitly holds up to Jagged Alliance.