Allow me to throw in my two cents on the subject.
There are gains and losses to be had on either side of the coin. The issues come from a range of fantasticallity! (not a real word I know) But anyway, with being the ‘only’ one, the rest of the world can seem rather ‘grey’ so a very carefully chosen powerset is almost required otherwise things become monotonous and too easy. So all I’m saying is that in the case of singular supers one must be careful that the world doesn’t lose its color.
The opposite can be said for the world of Superheroes angle. The world and all those in it can be so blindingly colorful that the main character can be swallowed up and nearly forgotten! Bad thing for any IF game.
Nothing wrong with either view its just a matter of how its put together. Personally I’d rather the two ‘mix’ in some form. Start off as ‘apart’ from the world and all those heroes and villains, feeling alone and then chaos!! 