"Okay... The boss in THZ3 should stay... It's like a challenge to hit him... "
Nah, it's a pain in the butt. There's a fine line between a challenge and a frustration and this crosses it IMO. Thing is, even if you know there's the danger of falling off and you take pains to aim carefully, you're still very liable to go flying off. Now, low walls in some areas and high in others, that would be a challenge :)
Couple of other things that got on my nerves more than anything else...there are way too many @#$%in' enemies in Castle Eggman! I can understand it's supposed to be difficult, but putting a mob of low-intelligence baddies in there changes the genre of the game to survival horror =P I literally entertained myself by lining them up and spin dashing into them to try to hit them all. The thing that makes this really annoying is that when you fall from a height out of spin, you know you're going to land on one, or else run into one a fraction of a second after you hit the ground, before having time to spin-roll, for lack of knowledge of the better term.
My prescription? Cut the number of baddies in half, at least. Perhaps make them faster to keep the difficulty where it's supposed to be. Change the command for spin-rolling from the -event- of pressing its respective button, to the -state- (i.e. if I know I'm gonna hit an enemy right after I hit the ground, instead of trying to time my press perfectly, I can just hold the button down and rest assured Sonic will roll into Señor crawla)
The one other thing that irked me was the Jetty-Syns. I do enjoy running from them, or 'jousting' as I like to call it, but after 10 or so passes without success I'd like to just not be attacked already. The problem, I believe, lies in the fact that destroying them is largely hit-or-miss. Making their collision mask just a bit larger would fix it perfectly.
/end wall of text.