I noticed that in the Scenario Editor, it is possible to unlock hidden units/props/terrain, whereas in the Definitive Edition it is not. Also, it is possible to place units very close to each other, while in the DE, they are forced to have a relatively big space between them.
Unit reaction time is better. They seem to react faster than in the Definitive Edition (build 38862). Archers, specifically, are the easiest to identify behaving this way. Villager decision taking seems also to be better.
If UPatch included attack move and stances, I’d probably play that version instead. And UPatch is not even being supported by a major company! I think it’s just a commited guy using his free time behind a screen. And even though the game did not receive an update for FOUR YEARS, it is in far better shape than the Definitive Edition, if you compare features and improvements side by side.