This game is great, no joke, i really enjoyed it and i think the main campaign is up there with the best Classic Sonic games, you got great level design, (mostly) great music, and five characters plus seven different powers, making it GREAT on replays.
The thing is, I feel like most people who played this suffer from recency bias.
After the main campaign you unlock a hard as hell (and honestly, at points really cheap) campaign with Trip, which can be at least numbed a bit by collecting the seven Emeralds in the main campaign and just avoiding all the cheap level design using Trip's Super Form. and then you get the Final Story, which isn't bad but it's quite long for a super sonic fight and a bit difficult to figure out (the trick is to avoid getting hit by the attacks, if you manage to at least consistently dodge most attacks, one of the other characters will show up to drop extra rings and give you more time, if you dodge all the attacks then Amy and Trip will show up to give you about 4 ring boxes), and then there's the co-op, which was never gonna be good. because unless they make it split screen (which honestly would feel like a nightmare), you cant have co-op work on a sonic game.
But that's the thing, all of those are things you're just gonna play once, you just need to beat Trip's campaign once, you just need to beat final story once, and you're probably just gonna do one run of the co-op to see how it is, if not just one session, every other time you replay this game, you'll play the main campaign, which is some of the best 2D Sonic this series has ever produced. This game is really good, but the problem is that it's surrounded by one-off stuff that left a lasting impression on people and replaced any goodwill the main campaign may have created. It's not a BAD game, people saying that clearly have never played an actual bad game in their lives, heck i wouldn't even say its an AVERAGE game.
Honestly if you beat the whole thing and feel negative about it because of the Trip campaign and Final Story, I'm begging you, do yourself a favor and replay the main campaign
play with the characters you want, explore the stages, use the Emerald powers, have FUN, remind yourself of the fun you were having in the main campaign, and see how now that the cheap stuff is over, you'll never have to replay that, you can just boot up the main campaign on replays and have a blast.
I said it before and I'll say this again, this IS NOT a bad game, it's no Sonic Mania but it's also no Sonic 4.