His tail hurtbox issue is almost like a placebo, a scapegoat. Yes its worse than smash 4, but that's not the biggest issue. The only reason people think its so bad is because its the most obvious. M2 was arguably the second worst off thanks to the airdodge change (jpuff suffers the most). Dtilt lost a lot of combo potential on its tipper hit, nair is too small and inconsistent and usmash has a small hitbox.
You need to look at it this way. In smash 4 if you won neutral just once of a dtilt, that could be 40% with a triple fair. If you then won neutral again with a nair, that was a guaranteed fthrow, so an additional 14%+. In this game, those conversions don't work which means that in smash 4 if you won neutral just twice, you had the enemy on 60%+. You win neutral twice in this game with a tipper dtilt and nair and you can get a whopping 15%. That's a huge damn problem when your conversions are so poor. But not only that, neutral was easier to win in Smash 4 thanks to the airdodge fair approach.
Mewtwo's buffs to fsmash, disable, ftilt, shadowball, confusion, dash attack, bthrow and weight can't be ignored, those are big. But the biggest issue he has is his conversion output.
The tail hurtbox has almost nothing to do with it because in smash 4, you weren't challenging an opponents dtilt with your dtilt from a range so far that the disjoint mattered, no one did that with the intion of actually starting a string. You can't just assume a situation where lucina or something is poking you from like 8 character lengths away, and that your response to that was dtilt to approach because even if you somehow clipped them, it required absolute frame perfect timing and the distance was so far, there wasn't any follow ups anyway from the max range dtilt.
Mewtwo has weaknesses, so what, every character does. But you are all doing yourself no favours by crying about the tail hurtbox all day long where even if it was fixed and in fact made BETTER than smash 4, mewtwo would still have the exact same problem he does now which is poor conversions. Put your effort somewhere useful.