Well, I wrapped up grabbing the red rings and doing the Hidden World+Nightmare Zone. And yeah... this part of the game doesn't quite come together as well as it should at all, and has probably damaged my net opinion on the game.
First off, the Nightmare Zone is crazy bland, just a quick boss rush with simplified versions of the first fights with the Zeti. It looks kind of neat, but that's about it. And the hidden world is just plain crappy. The first zone is stupid and doesn't give any indication of how you're supposed to "beat" it. The second is just plain flat and dull. The third one is... okay. But you pretty much need to go crazy trying to grab the rings to make it interesting. And the fourth is just awful. Really, really... bleh. Poor idea, and a really stupid way to "end" the game (fortunately I decided to save trying out Super Sonic for after this, and that was at least kind of neat, even if I'm not sure he really meshes well with a lot of the game design).
The red rings kind of fall into a general issue that's probably the root of damn near all the problems I see in the game: they really didn't do much of anything in terms of alternate roots. Instead, we have "alternate" routes provided by running to the other side of the tube, or what would traditionally lead to an alternate route ends up as just a shortcut or one of those gold cannons (which feel extra pointless). As a result, the red rings are either left right in front of your face and super obvious (I recall this being especially prominent in Sky Road 4, where I had no problem getting them all on my first try), or they're made artificially hard to get due to auto-run segments (which often have a checkpoint right after them!)
The biggest problem with the wisps (outside of some of them being slippery) is related as well. They went from being a crucial element that opened up paths to just kind of being... there. For the most part, they're completely unnecessary and just act as a minor, flat shortcut. They even made sure you would pretty much never die with Drill by placing respawning Wisps everywhere you can use it. And then on top of that they seem to have completely ignored the ability of players to hold on to a wisp and use it elsewhere. Hell, I realized the reason I found the Rocket to be so janky in 3-4 is because they clearly expect you to always just fire it where you get it.
Oh, and Black Bomb sucks to use (took me forever to just kill one enemy with the explosion, and I'm sure I'll never do the mission for 3),and they REALLY should give you a score at the end of the level in normal play like they used to (unless I'm missing something. I'm guessing it's left for Time Trial or whatever?)
But yeah, as a net whole I still think there's a lot of good in the game, and it's still a hell of a lot better than any of the stuff that came between 3&K and Colors in my book, at least on consoles.