I kind of feel MadeMan on this. Gens is a game that honestly feels worse from a certain perspective every time I play it. The level design is kind of garbage; the reliance on boosting and homing attacking your way through the levels really shines a spot light on just how poorly Sonic controls in a 3d space as the game would be literally unplayable without those abilities. The bosses are mostly junk, the story is barebones, and the level selection is morbidly bad. The music/visuals are nice and the game has its heart in the right place but from a gameplay perspective it is certainly not the messiah many make it out to be. Far from it. A game where Sonic controls so poorly as he does in the boost games fundamentally is broken.
Another thing to note is that context is everything. After monstrosities like 06, Shadow, Black Knight, and Unleashed, Colors/Gens look good by comparison. Really damn good. But if Gens was the first game you've played since the megadrive games or even SA1, you might not be impressed. Most of us had the luxury of having played Unleashed first to ease us into the boost style of game too.