I personally think it's well designed. In fact, I can only think of one spot which I found unfair : the sliding section in part 3 of Pencil Pentathlon, where you have to walk down the slope and not run, otherwise you'll jump too far and die, and there's no way to know that beforehand.
Everywhere else, I felt that the game did a good job of explaining what to do, even if some sections were difficult. Which makes me curious: do you guys have more detailed examples of some of the parts you found unfair? It's been a while since I played the game, after all.