Spider-Man also has an unluck disadvantage. He gets colds, loses Aunt May's birthday presents, gets accused of all sorts of crimes, and otherwise has a miserable life, but not because he rolls 1's on his Wild Die.
Most superheroes are VERY competant ALL the time. If they fail, it's because the other superhero did better, not because they dropped their shield, twisted their ankle, threw out their back, or otherwise fumbled. Think about it. How many superhero games have such a mechanic? Most of them have a mechanic for improving results (Hero Points, Hero Dice, etc.), but none of the good ones, anyway, have the fumble mechanic. How does such a mechanic help emulate the genre? Have you ever played a superhero game and said, "Oh man! This game would be PERFECT if only my character sucked more!" With that mechanic, you'd probably end up having players blow Competency Dice to compensate for the -1d6 to their roll. That would be a shame, as Competency Dice should be used for cool things, not to mitigate penalties.
Just for kicks, I picked up 10 random comics from my collection, and flipped through them. I saw examples of what might be called failed rolls, but SUPERS! already accounts for that. Events that qualify as fumbles? None - and yes, some of them were even Spider-Man comics (although I know that he would qualify for quite a few).
Now I'm laughing at myself for going off on this so passionately! Please, don't take any of this the wrong way. You just caught me when I'm sleep deprived.