I truly think that in a vacuum, USUM are decent Pokemon games, not high up there but they aren't bad.
It's just the fact that they did so little to warrant costing 40 bucks, and they really didn't do that much to the base games to warrant being the definitive versions, is what makes them bad in a lot of peoples eyes.
The things that USUM did, new/differing events, new dialogue, new areas, even just little things like differing camera angles from the base games, really help a lot with making Alola an enjoyable adventure. And it's really obvious when playing SM and USUM back to back. They should have just released USUM as the base games and left it at that, maybe add some free DLC with more content later down the line make sure Gen 7 stayed alive until this point.
But despite all that, it wouldn't have fixed what Gen 7's biggest issue was, the region was just so bare and empty. Every town, dungeon and route felt more like hallways than places. And that puts a damper on any Pokemon game.
