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The problem with USUM is that they're the same as Emerald in that the vast majority of the game is the same. Aside for a couple fights with the albinos, the game doesn't take any sharp changes until you reach the Aether Foundation. Which is at the half way point of the game. Which means for half the game you're doing the exact same shit you were doing before. Now the changes themselves are...okay. It's a lot more grander than the original games. It's less "fix Lillie's family" and more "save the world from prism rapists."
I think the worst disappointment of the entire game is Ultra Megaopolis. The trailers really hyped up exploring the Ultra Space only for you to get there and turns out the entire city consists of a single hallway and a set of stairs. Ultra Necrozma itself was great. Hype all around. And right as you start having fun it ends and it's back to what you were doing in the first game. Well I'll be fair, Hau grinding matches all over Alola so he can face you as Champion was a nice realization of his character. Lusamine is a lot less of a cunt which I haven't decided if I like or not, she was a good villain in the first game and while she's not necessarily heroic in this game she's definitely not the villain anymore.
I think the core problem is that BW2 exist. BW2 and even HGSS showed that Pokemon sequels are far and away better than remakes. So when USUM came out as a remake we were already past it. It's probably great if this was your first game. But if you played it before I can't argue against feeling like you got gypped.