I still consider it to be mean game design, and it's one of the reasons that, if I do decided to play these games at some point, I'll start with Secrets of the Silver Blades.

(Pool of Radiance's level cap is too low, particularly for Clerics who don't even get the second healing spell (and the first one is *really* weak), and there's no healing shortcut to mitigate that issue the way there is in later games in the series.)

(Also, when I do play, I will be hex editing my characters a little in order to bypass a sexist rule that should not have been in the game.)

I agree. I don't have as much of an issue of the "trapped in a dungeon with no place to train" mechanic as the "drop all extra experience when you do train, leaving you close to the next level up". Combine that with how dual-classing works and the exponential XP requirements to level up, and it's just a bad combination. They should have left the XP al;one at training. Maybe make a mechanic that you can't train again until you get at least SOME XP at your current level, but even say 37EXP and then you can go back and take another level-up. Or better yet just let you take all the levels you have the XP for at once.

At one point playing CoTAB I was frustrated because I'd just duel classed and was at level 1 but had enough XP to get back to level 6 (when I dueled at level 10), but lost all the XP. I just wound up using Copy II+'s sector editor (precursor to a modern hex editor, only it's ordered by what's stored on the disk instead of following files. Great for non-standard OSes like the RDOS used by many SSI games on the Apple II) to just cheat my XP back up to what it was before I trained to level 2. Then trained, saved the game, rebooted into C2+, reset the value again, and repeated until my XP value was left alone.

Worth noting that the mechanic of not being able to gain multiple levels at once makes more sense when you get to 3.5e D&D rules, where the XP you gain from a specific enemy decreases as your level gets higher. Without that mechanic, and without automatic leveling, you could just hold off on a level up and gain more XP, allowing you to level up faster. Then again, by the time this became an issue, D&D CRPGs would generally allow you to level up anytime outside of battle, so the main issue that plagues CotAB doesn't actually apply.

(By the way, the 3.0e mechanic, as seen in Icewind Dale 2, is fundamentally broken, especially in a game that allows you to create new characters mid-game. A high level character will gain more XP from an encounter if accompanied by a low level character.)

Also, Wizardry did not do the "no multiple level ups" rule; if you got enough XP to gain multiple levels, as could easily happen right after a class change, or if you're using experienced adventurers to train somebody new, or even if your level 1 character managed to solo a group of creeping coins (not that hard if you have enough HP and AC, but the other encounters there are a problem), you could just keep resting until you gained all the levels you've earned the XP for. (This could also happen with Wizardry 1's identify glitch, or with Wizardry 2's class change items that don't change either level or XP, in a game where XP requirements vary by class.):

Might & Magic also has no such rule.

Some RPGs, like the original Final Fantasy do this, mainly because the developers didn't expect you to get enough XP for multiple levels from one battle. Specifically, you level up automatically when you get the XP needed after a battle, but you can only gain one level per battle.

IIRC M&M was similar to Wizardry. You could only gain 1 level at a time, but you could keep training at the training ground (and paying the increasing fee. It maxed out at 65535) until you had received all the levels you were entitled to for your current XP.
THAT is how I would have appreciated the Gold Box games implementing level ups.

That cap doesn't apply for most of the series; in fact, it's probably just MM1, which is the only early MM game (MM5 and earlier) that I haven't played.

Also worth noting that, starting with MM2, the series got rid of one issue that plagued Wizardry and the Gold Box games, and that's random level up bonuses. (In the Gold Box games, it's only HP that's random, but that's still annoying.)

In MM3-MM5, the cost to train increases quadratically, whereas the XP needed to level up stops increasing around level 12. This creates a rather frustrating issue in late MM5, especially if going for the World of Xeen ending: You have enough XP to qualify for dozens of levels, but you don't have the gold to actually gain them. Even worse, the game keeps showering you with XP (which is useless without gold), but is rather stingy with gold at that point.

M&MII has the cap on training costs, as well as random HP gains. I'm pretty sure it's a die roll based on your class similar to Dungeons and Dragons. I know little about Rolemaster (I think that's what M&M was based on the way the Gold Box games were based on AD&D).
You COULD go to one specific location in a castle basement (I forgot which one, but it's in most guides) and pay an exorbitant fee to max out your HP.
As for the cost of training, I'm pretty sure it increased quadraticly or exponentially until it hit the largest 16-bit integer and was then capped.
I haven't gotten around to playing M&MIII or later. I purchased it right before the pandemic shutdowns and then Epic started their weekly giveaways, plus some relatives started cleaning out their home and offered me some old games and I never got around to playing the rest of the series. I haven't really purchased any games since then due to a combo of the giveaways from places like Epic and GoG, and the stuff I got from my relatives. I've got a backlog of stuff to play, and I also enjoy some old classics. Since the pandemic ended I've had less time to play than I did when I was a kid or even a college student going to school and working part-time. Real life responsibilities like maintenance of my home, grocery shopping, etc.

No, HP gains aren't random in MM2; they're based on where you train (along with class and vitality, of course). (Note that it costs more to train at places that give you more HP.)