every other super is completely avoidable being hit by on reaction after super flash either blocking OR jumping. Mewtwo's is not. Which means you literally are helpless once he's in close. You jump? Wait til about to land super. You are blocking, super. Every other character the worst you have to fear generally is a grab though those hurt like hell too, they are not a potentially 395 damage move
Gengar's super is also a grab that's unavoidable if you're grounded during the flash, actually. Dealing with grab supers once they're in is a guess, just like dealing with normal grabs is a guess, just like dealing with strike supers is a guess.
Yeah, you can sit there blocking and if someone without a grab super throws their super out randomly you're safe. But if they walk up and grab you twice in a row you've probably taken nearly as much damage as you would've from a super and you still have to worry about super the next time. It's literally the exact same mixup, the risk/reward is just weighted towards the grab instead of the strike.
It also has significant startup before the flash; it's not as simple as "see someone blocking and super."