The story is dogshit. It's a very basic plot that suffers from presenting itself as being more than it is and having a shitton of repetition across different campaigns. Furthermore, half of those campaigns are outright padding with no bearing on the actual plot.
Sonic: Has the overall best campaign in the game gameplay wise, though this isn't saying much. The game's controls are a little too precise for how fast Sonic goes, but I feel like the devs knew this, because his story is really, REALLY easy. While there are momentum based physics at play, the level design very rarely gives you meaningful opportunities to utilize them. Story wise I greatly dislike Sonic in this game. He's a bumbling idiot with barely any of his trademark snark; he just comes off as a really generic protagonist.
Tails: Follows Sonic around for most of the game, so his story is kind of an abridged version of Sonic's for most of it, in both story and gameplay. His levels (except his last) are just shorter, neutered versions of Sonic's (although in Casinopolis and Sky Deck's case this is a good thing), and his story is just Sonic's again but from Tails' perspective. In fairness, I prefer Tails' perspective, because unlike Sonic, he has something of a character arc in learning to be more independent, which culminates in his last stage, Speed Highway. Tails' Speed Highway is the best stage in the game in every way. You must skillfully utilize Tails' flight ability, you're racing Eggman instead of Sonic which makes infinitely more sense story wise, and getting through the stage quickly requires mastery of the controls. Never in any other part of the game does it even come close to touching this level in terms of quality and that's just sad.
Knuckles: Is my favorite character to control, but is sadly stuck with mediocre at best levels. Treasure hunting isn't an idea I'm inherently against for Knuckles, but the level design isn't built around him; it's just repurposed areas from Sonic's story. As such, exploring them as Knuckles doesn't really feel that great. Story wise, I'm not a fan of him either. Like Sonic, Knuckles is a bumbling moron. It tries to paint it like Knuckles is dumber than the rest of the group but in actuality Sonic does shit that's just as stupid so I don't buy it. Either way, watching idiots be idiots the whole game isn't fun, and it doesn't help that Knuckles doesn't get many chances to be a badass like he should.
Amy: Filler character. Something's wrong with her physics because she just cannot go fast ever. Her level design is generally pretty bad and consists mostly of rudimentary platforming and "puzzles" an infant could solve. Story wise she has no reason to be here and no character at all. She has always been my single least favorite character in the franchise, and this game does her no favors.
E-102 Gamma: Filler character. Like Amy, he has no plot relevance at all, but his character arc is better than most in the game. It's a pretty typical "robot learns what emotions are" plot which I honestly really don't like but it's more than most characters in the game get. Gameplay wise his stages are a combo of platforming and shooting combat and tbh they're mostly not bad. It's fun to try and target as much stuff as you can and shoot it all without stopping. His level design can get somewhat creative on occasion too. This is my 2nd favorite campaign.
Big: 0/10 do I even have to say anything