I agree with you to an extent mercilous. His durability is a big factor no doubt, his durability is nearly as unlimited as Hulk's strength, but I wouldn't say it's the only factor when dealing with other heavyweights. He has to be strong as hell to even be a match for someone like Hulk, otherwise he'll be the indestructable guy that gets slapped around all the time. I'm not overestimating Juggernaut, but to say he can't dent Ultron like Hulk could is kinda an understatement, cause he definitely has the potential for it.