Well it's more than that. I'm not sos sure how much you know about competitive smash but in the competitive environment he has a lot going against him besides being slow he is big (wich means he is easier to hit) and light(wich means he gets k.o.ed easier) those two things are a bad combination.
To help you understand what I mean Just imagine if bowser kept his size but was light as Kirby. See where I am going with this?! And that's not all. There are many other reasons aswell
Also I'm not 100% sure if Roy or Ryu will come out as DlC but there has been evidence of there existence in then game. Some hackers have found sound files in smash 4 of Roy and Ryu's victorie themes. They know this becuase of the sound files titles. I don't teme the names of them they where something along the lines of toys victory theme and Ryu's vicory theme.
Some people think that's A hint of future dlc I am one of those people.
Edit:
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got to this page and read mewtwo a attributes particularly the section that brings up his weaknesses if you are interested. These are still weaknesses for him since he is virtually the same as he was in melee.