I liked the Team Rainbow Rocket, but it feels like something they wanted to do for Anniversary 2016 and didn't finish in time.
The bosses in USUM were pretty good.
"""New Areas""" like Pikachu Valley and Ultra City were way too small. Compared to Distortion World or Battle Frontier which were massive additions.
Narrative changes were kind of weird and soulless. Lillie and Lusamine resolve their broken mother-daughter relationship off screen lmao. It was better in the vanilla game. Also the dad comes back, and there's just no emotion when they find out that he's alive? The anime of all things rewrote this whole subplot.

Overall USUM is the weirdest "3rd Version", where they fixed a few things and left others still feeling unresolved. Unlike Platinum which consolidated DP into one game and expanded on the lore. I guess Iwao is more of a boss battle guy than a storyfag.