The fact that XP doesn't carry over from one level to the next. So when your unit has say 70/100 XP and it gets 100 XP in a fight, you start at the next level with ZERO XP instead of 70. I have literally no idea why the designers of this game thought it was a good idea.

It's a bit annoying, indeed, especially when you're trying to look for some battles where you won't waste a decent amount of XP because your unit/hero is going to advance.

System of "levelling" units/heroes is still improved comparing to first Disciples (less improvements for units, when unit obtained the maximum "transformation" on the level it couldn't get anything from next fights), but yeah, this one little thing and it would be perfect.