I see what the combo-by-design mechanics you speak of are, and I'm not really a fan of it.
The levels are really really big, and while it shames me to get a 20 min completion time on some of them, I find some of their gimmicks a bit too overcomplex for their own good (like unlocking the doors in the ruins stage, something that's done way better in a later stage, or destroying the four ships in Sky Battalion). I know there's secrets to subverting some of these with certain characters (keep in mind, I've only played as Lilac as of right now), but I'm not really seeing a lot of branching paths, which I'm hoping are as cleverly hidden as those in S3&K.
The freedom of the levels and how quickly they pass by is directly countered by the slow, methodical and often precise nature of the bosses, which is a little weird. They also seem to directly counter the movesets of Lilac on a lot of occasions. Also, the physics are laughable in this game, with not really all that much momentum going on and very sticky characters. The level themes are a bit wonky (as in, their aesthetics), I'm not that much of a fan of some of the overcomplex mixtures of concepts like Asian/Casino Night, but most of them aren't too weird. I will say that the grunt unit found later in the game that takes a number of hits to kill and fires numerous shots is an annoying enemy that soaks up a lot of life points- which is one last thing I'm not sure I'm completely sold on.
If there's anything I can say, it's that Freedom Planet takes some pretty direct queues from Sonic, but it definitely feels like it's own game, and I really appreciate that.