>>35546390They changed the storyline a bit, added new formes for Greninja, rock doggo (these are event-only and can't be received anymore) and Necrozma. They also added a minigame to catch legendaries and high level Pokémon.
That's it.
It's not worth it if you already have Sun or Moon. It's very mediocre as mainline games.
People mock Diamond and Pearl for being slow, but USUM are way worse in that regard. Especially if you have one of the original 3DS models.