Mary Sue as in perfect in every way. Now if they fail, sure that’s a story but by definition MS can’t fail. That’s why they are story killers.
There may be some semblance of a struggle but you as the reader knows ultimately there’s no real danger to the MS. Failure implies growth through learning from the experience.
I love Superman as a person but as a character he’s boring. So boring the writers had to artificially create weaknesses. People seem to like Batman more since he’s more vulnerable but I think even Batman is starting to move to MS territory. He went from a clever man with ingenuity to the most smartest man in the universe. Even Batman is getting boring.
