I think MUA3's combat is technically better than the previous games (aside from some balance quirks and the over-emphasis on synergy attacks), but it feels a lot less like an RPG compared to earlier titles.
There was plenty of jank before, sure, but being able to go more in depth with builds and skills/abilities for each character is seriously missing from this one. The incredible roster in this game barely has a chance to do anything with the four-ability limit and questionable assignment of passive abilities. No more messing around with fun, one-off skills that have unique mechanics (playing as Dr. Strange and Nightcrawler in MUA1 is a far cry from what we have now, for example); no more need to carefully consider how to level heroes; no real flexibility to create novelty, challenge, or broken builds; etc.
When I play X-Men Legends and MUA1 (and I suppose MUA2 to a lesser extent), I'm reminded more of a light Diablo or Gauntlet Legends. I don't get the same feeling when I play this one despite every game appearing quite similar at face value. MUA3's excessive loading and heavy emphasis on multiple modes may have something to do with that missing intimacy, but I think it mostly comes down to the way character building has been simplified.