I don't think its because they are "overpowered" per say, but because they are just binary characters in general
Superman for example, is pretty much incapable of receiving any worthwhile damage to his body unless he comes into contact with a glowing green rock. like that is literally the only thing that can stop him from being able to uppercut you into orbit. so how do you make superman a character instead of a cardboard cutout of a man? you can't exactly put his life in danger, so you have to put others in danger, making it seem as if he's too perfect to ever be realistically threatened, and he can only be defined by other people suffering.
Lets compare him to my favorite superhero, Spiderman. Spiderman can be defeated in a variety of ways. you can shoot him in the face, beat him to death with a blunt object, stab him, ect. Even though he can shoot webs from his hands, he's still human, he's still fragile. His characterization comes from him having to balance being a superhero and a regular guy, his struggle comes from having two identities that require totally different things from him. He can't let Peter Parker become Spider-Man, because he wants to live a normal life, but he can't let Spiderman become Peter Parker, or his life would always be in danger.
Peter Parker BECAME Spiderman, but Superman became Clark Kent, if that makes any sense
I don't know if the same thing applies to Captain America, but he is also pretty binary, can my shield block it?
>yes: I win
>no: I lose
His characterization is... I dunno, he seems to just be GI Joe all the time, not much of a character there. Maybe they should have a section about him trying to live a normal life and how that doesn't really work with him, that could change people's minds about him