I'd say probably my biggest nitpick is the often uphill stage design. I've gone in depth regarding this topic in other threads, so I'll try not to go super in depth here and just cover the overall point, but Sonic stage design generally functions best when the overall trend is for the player to be led downhill so as to be able to take advantage of the speed this makes possible if the player so chooses. Sonic's speed is usually capped while running, so the player can stop themselves from going too fast if they desire it such as playing cautiously or exploring for goodies. However, by curling into a ball the player can take advantage of big bursts of speed, and in some cases can even avoid uncurling for the entire stage.
SRB2's stages on rare occasions have setpieces that feel designed like this, such as the big ramp near the end of GFZ2 leading into the big open area, but the general trend is for the player to have to platform upwards on their journey to the goal sign, a trend that advantages vertical playstyles such as Tails and Knuckles.
The way I think about it is that Sonic stage design is similar to designing a roller coaster. Uphill sections do exist, but they serve the purpose of building up anticipation for the next speedy downhill section. You don't typically end a roller coaster ride with an uphill climb, the finale is a final overall downhill rush of speed designed to be the most iconic section of the ride.