Only played a 'few minutes' trying it. Notable so far two major bugs.
Edit: My previous version was 2.4
Edit: its now down to 'one major bug' and some others

--

The infamous end turn game-hangs bug is still there. I click the button for the next turn and it eventually stops responding. The odds of this happening depend on the game settings and game progress and probably mods used. The 'more' is in the game the sooner and more often it occurs. Many 'insane' AIs on hugest map works well for reproducing it.

Edit: My impression is that it is worse than before but no hard evidence.
Edit later: New impression is that it is not worse than before. It is possible that it is better at least in the sense that I do no longer need to screw around with process/or affinities like before. So it may be that some multithreading bugginess was solved or improved. But it didnt fix this problem. These end-turn hangs still happen as described.

--

Edit: This was a user error. Custom resources still work.

Edit: Will continue to try 2.5.1 for now.

GUI(?) updating is still bugged at least for tax overlay. If I place a champion in a town and give it unrest reduction on level-up it will not update the tax overlay unless "something" happens like me moving the champion out and then in again. Dont know if this is just a GUI display thing or if taxation is also wrongly done.

Test game, first battle. Killed some creeps, clicked on town to enter but no moves left. Clicked next turn button. Got attacked by other creeps (spiders) and they killed 2 of my units in tactical combat (not champions but normal ones). Game moved my stack into town automatically. When I looked at town I see the dead units still alive with 0 HP. Next turn they started getting HP back. This is probably not a new bug. I have seen previously slain units stay alive before.

Pathfinding is still buggy.

One example for the overlay pathing - not the actual movement but the display of the "best" path when mousing over: At turn 1 or 2 in a game after founding first town close to an existing road: Somehow the pathfinding didnt consider the better movement on the road and couldnt find a (very obvious) shortest path which was ~2 tiles away. Manually moving onto the offending road however worked though. It only costed the reduced movement points allowing me to move one tile further than the overlay path had susggested.

Another example - also the overlay: Road building costs movement points but the overlay doesnt consider that. It somehow remembers the value from before building the road or something. This can stack up to some degree leading to quite a mismatch between what the overlay thinks is possible to move yet vs what is actually possible to move yet. Only once movement points are down to 0 its usually correctly updated.

--

Added the following to first post about end turn hangs: Also considering how easy(*) it appears to be possible to reduce this problem (but not fix it) it should be trivial for a competent programmer at Stardock to massively improve the behavior.

(*) Zoom out map fully seems to help a lot but doesnt work 100%.

'Fluff' bug - not a technical one - and not 100% sure this is not a mod based bug. Plague in capital quest: The plagued population quest site isnt built near the capital but near the sovereign even if sovereign is 100 tiles away on the other side of the map.

Another pathfinding issue: Flying units pathfinding thinks it can move over creeps but when the movement goes to that tile the flying unit stops instead of moving/"flying" over the creep.

More GUI(?) updating bugginess still in the game: Like I cast production enhancing spell on town while the town detail view is open. This leads to two different production times being displayed on the same screen. One is updated and the other unchanged == bugged.

Summon Spells by creeps dont seem to work anymore. This is mostly new. I think this occasionally happened before but now I have never seen one working. It may have to do with the AI trying to cast it onto a hostile unit; this is the same behavior as always though so it may be a code change which causes it to not work anymore.

Dont know if this is a problem in base game or caused by a mod.

Edit: I have seen it at least for 'Wildling Shaman' (multiple spells) and 'Haunter' (Summon Plaguestalker).

Edit 2: The following links suggest that this is not a mod problem but a game problem:

https://www.gog.com/forum/fallen_enchantress/lh_20_xtradeconstruct_legacy_of_magic_mod/post45
https://steamcommunity.com/app/228260/eventcomments/1644304412654080783/#c3356799628310444384

The tactical combat gets stuck bug may still exist. I just had a potential instance but not 100% sure because I did some modding for testing.

About the "The tactical combat gets stuck bug may still exist. I just had a potential instance but not 100% sure because I did some modding for testing." and the AI summon spell targeting bug: I just modded to use this and in a tactical battle with many casters the AI still always targets hostile units and not empty tiles. Interesting here is that the tactical battle bugged out with the game hanging (old bug sort of); analyzing the gfx suggested that somehow the game had summoned multiple units onto the same tile(*) - which may cause the hang; anyway my impression is that there is (apart from the tactical AI targeting non-empty tiles) at least one more bug - unfortunately I cant say for sure what exactly it is as I dont think the gfx can be trusted fully

(*) probably from multiple castings not from the same casting

At least one clear cause of the hang in tactical combat is pathfinding - in tactical maps with many obstructions, if an AI unit can't find a path to reach its preferred target, it never stops trying, causing a hang. I see this often enough that I always quicksave before combat now.

Two solutions are obvious:
1. For the programmers: Add a timeout/limiter to tactical pathfinding, after which just "pass" (or choose another target).
2. For the user: when hung in this way, auto-resolve does still work.

One other bug noticed in tactical combat: in rare circumstances the AI can move two units into the same square. At least, I assume that's unintentional. Surprisingly, it doesn't seem to cause any problems.

Cannot load a save if I quit the game and come back.

I'll start a new game, save it and then shut down the game. When I go back later and try to load the game, it crashes back to the desktop.

The only workaround I've found is to start a new game and while I'm in the new game, I'm then able to load the other save. It's the only way I can get access to any of my save games.