I think your definitions are askew. Challenge should indeed be in regards to how well players are capable of dealing with mechanics and game elements in varying situations that are clearly communicated to the player, however in most cases the best form of communication is to just have the player experiment with the game. Lest you wish to have tutorial screens pop up, or force the player to see certain things just so they have a full grasp on how stuff works. This is the thing with Sonic in comparison to other platformers. Rarely, if ever, should good Sonic levels dictate pacing. The game's all about flow and the player's ability to properly take advantage of their abilities to keep the momentum going, where a skilled player can blast through even a seemingly "sluggish" stage. If the game constantly tries to stop you and hold you back to get you to see this new mechanic that they need to teach you, it ends up feeling stilted and broken up. This is why the original games do clever and subtle things with their levels. Placing rocks in your path where an enemy or shifting platform might be coming up. Using a humble ramp to slow the player down just enough so that they can react to an oncoming obstacle without completely breaking the pace. Having a hill or slope encouraging the player to roll along them and take out some enemies near the bottom. The game does it's best to give players the heads up necessary to deal with oncoming hazards without being intrusive. In all fairness Mania doesn't do this as elegantly as the originals. There are a lot of times where they sort of stop you with a rock or platform so that you can see how a badnik or gimmick works, and there are cases where the game does an absolutely horrible job at conveying things to the player, mostly in some of it's boss fights. But the level design has an unprecedented sense of flow and consistency to it, and the enemy placement is quite honestly some of the least offensive I've seen in a Sonic game. The mechanics are all great and very easy to grasp. It's not perfect by any means, and I'd argue Sonic 3K does it a little better, but given this was their first shake at things, they did a pretty frickin' great job. I'd rather the game be a bit tough, but doable and fun, than overtly forgiving and bland.