i don't think i've ever gone back and forth on a game like this before. if there's one problem with the way they chose their levels, it's that the last... two thirds of the game, but especially the last third, have this form of schizophrenia. the first third, the genesis era, is great because the levels kinda work in that order: green hill zone was first in sonic 1, chemical plant was second in sonic 2, and while sky sanctuary was the capper (or close to it) for a lot of stuff in the genesis era, it works here to help ramp things up. speed highway is something from some point in sonic adventure, so it's good as a middle zone too. then it's followed by two introductory zones from sonic adventure 2 and sonic heroes respectively. those levels were meant to be easier than the rest of the game, and i think it clashes with the attempt to put them in the middle of the game.
the first two thirds of the game are fine for me overall though. crisis city is just this ugly thing, and rooftop run is the fourth introductory stage in the game, except now it's near the end of the game.
and planet wisp. holy shit. part of what made sonic colors so great is that it's a straight up sonic running to the finish line thing until you unlock the wisp powers, and then you have the option to use them. iirc, by the time you hit planet wisp in colors, you're not spamming colors powers all the time. and then there's all sorts of weird shit that didn't even happen in colors's planet wisp like falling blocks or the gear charging thing. it's like no one from the colors team was available to help out. definitely the biggest disappointment for me so far.
the ramp up to the finish doesn't really work. sonic keeps learning new moves and techniques, but they're only applied to certain acts and not kept throughout the game. the light speed dash, drifting, colors powers, and stomping (well, at least how it was used in crisis city act 1) didn't need to be there. i also think the reason the genesis stages stand out so much is because they're inspired by things with way better level design than most of the rest of the game.
there needed to be a better way of picking out the levels, especially on the back end. at least, the way things were arranged feel wrong because of the design choice to separate the zones by eras.
also, it feels good to actually talk about a sonic game needing better structure than HOLY SHIT WHAT THE FUCK IS WRONG WITH HAVING THIS BAD GAMEPLAY THING THAT IS CLEARLY BAD. it's a considerable improvement.