Difficult stages in Sonic 1-3&K were a satisfying sort of difficult. Difficult Stages in Unleashed were an aggravating kind of difficult, mostly when playing as the werehog.
I think it was the sheer amount of time that it took to play the werehog stages that made them so tough. I can handle a good 8 minutes of werehog, making difficult jumps and taking out a few hard enemies, but when stages hit the 20 minute mark (they were so fuckin crafty to remove the timer in the werehog stages) I feel like I've had enough. Case in point, I played Jungle Joyride night a few days back and wasted 15 minutes playing through the stage, only to be locked in a room with wave after wave of enemies coming at me. I could have "givin it my all", but at that point I just wanted to play a daytime stage... I was tired of punching, I just wanted to run and jump. So I shut off the 360 and played Sonic 2.