I'd rather have a normal counter than Disable. Its better in every single way.
Nope. You are comparing apples to oranges. It would be awesome if Mewtwo did have a counter (or some kind of similar defensive measure) but disable is designed to be purely offensive measure, and it is awesome.
Without disable, the only time you can get a fully charged smash attack to connect is when your opponent is a total noob. With disable, you are one of the only characters (before bayo, there was none other) who can justify charging smash to full, and lets not forget about how it opens up taking stocks at 70%.
Its hard to land, but Ive been practising my SH>AD>FF>Disable timing which is a great mixup. They like to drop shield and dash forward when they see that I have "mistakenly fallen a bit short". Then they realise just how much range disable has, fall asleep, and subsequently die, mashing their control pads while screaming "WAKE UP" at their unresponsive avatar. UpTaunt.
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Besides... It seems like everybody has a counter... its so mundane. I would rather have rare and unusual tools that are difficult to use but extremely effective when you use them right (I suspect this is the mentality of most Mewtwo mainers)... If I feel I REALLY need a counter, then I switch to Lucina.
Too bad DLC custom moves never emerged. I was really looking forward to collecting all those, and a counter would have been a good one to include... even if it were just a super-armoured move instead of an invincible/intangible one.