Honestly, I would say falling off a cliff and dying is bad design. How many cliffs can you fall off during sonic games nowadays? It's just a million cliffs. I don't like to be killed instantly by anything in a video game. It's the ultimate punishment a game can dish out, it usually happens because "you didn't realize you had to push 'A' fast enough".
Obviously they had this in the Genesis titles as well, but it wasn't the source of the majority of my death. Colors (while a decent game) was a bunch of tiny platforms hovering above nothing for half the game, and I constantly lost ten lives on the same pit. I haven't played any other "new" Sonic games really, but from the sounds of it, they're worse than Colors. I assume that kind of shit was in there too.