Is that they actually don't know superman. Like they think he's simply invincible, that he can't be beaten without kryptonite, that kryptonite is his only weakness and so on. Or they just think he's "OP" and make the claim that his strength means his stories are automatically boring. When in actuallity he has several weaknesses, and is very much beatable. Sure he's strong, but the people he fights again a hell of alot of the time are just as strong, if not stronger, than the Man of steel. Oh and I forgot, another reason he sucks to some people is that "he's pathetically weak" though you'll only hear this from Goku fanboys :P