Mewtwo is a high-risk, high-reward character in a very similar fashion to Ganondorf. Their key strengths lie in their range, and power. But they are also similar in that they have very exploitable weaknesses to counteract their massive strengths.
For Ganondorf, it is speed and recovery (though Wizard Dropkick helps in that regard). He is slow, and has pretty slow framedata as well. But when he hits you, he hits like a truck filled with dynamite. In contrast to Mewtwo, he has more power and priority.
For Mewtwo, it is his extremely light weight and somewhat slow frame-data (though not as bad as Ganondorf I think). Because of how light and big he is, he is very susceptible not only to combos but also to being KO'd very early. However, when he hits you, he hits like a truck...minus the dynamite, since Ganon has the edge there. However, in contrast to Ganondorf, he has more speed and much more recovery. One of the best in the game, I would say, and he can go for very deep when it comes to edgeguarding with his aerials. Plus, what he may lack in priority in comparison to Ganondorf (I think), he has more range than Ganon as well thanks to his tail and having a projectile.
Both of them have a command grab that, although they might not be the best moves in the games for
true combos, lead into a lot of situations to try to mix the opponent up. The difference is that Ganondorf's Flame Choke leads into tech chases and keeps the opponent on the ground, and Mewtwo's Confusion forces the opponent to burn their second jump in most instances which makes them more susceptible to other things like Fair/Usmash/etc. Both moves also aid their recovery. Mewtwo's Confusion gets the bonus of being able to reflect projectiles, but loses out on chasing situations because Ganondorf's Flame Choke is more reliable and because of how some characters have multiple jumps.
Both of them have a situational move that can lead into a fast KO. Warlock Punch is incredibly slow, but has armor and a ton of knockback. Mewtwo's Disable, while fast, has very short range and only works on grounded opponents whom are facing you. Both of these are risky moves which can potentially turn into an early KO.
Wizard's Foot and Shadowball, while quite different, have somewhat similar applications. Both of them serve as long-range KO moves. While Wizard's Foot doesn't have to be charged like Shadowball does, it isn't a projectile. And Shadowball is stronger. Wizard's Foot gets the bonus of serving as a meteor in the air (or as a recovery if you use Dropkick instead), and Shadowball gets the bonus of that you can shoot it even when it is not fully charged.
Their recoveries are completely different, and serve to further benefit their own unique strengths. Ganondorf's obviously focusing on power (mostly if you are using Dark Fists though), and Mewtwo's focusing on speed and recovery.
Both characters are also vulnerable to rushdown characters such as Fox, Pikachu, Sheik, etc.
So while Mewtwo and Ganondorf are very different in their own ways, they are also very similar in that they are high-risk, high-reward characters. In summary, they are like opposites within the same archetype of character. Like they are some sort of Yin-Yang, with Ganon being the Yin and Mewtwo being the Yang. You play them differently, but their goals are the same.
All, or nothing.
You have all the capability to massively punish your opponent's mistakes with these two characters, but one mistake and you can just as easily end up in their position.
On the plus side, I feel as though Mewtwo will probably be more competitively viable than Gandondorf overall. Because in fighting games, speedy characters usually always trump slower, bulkier characters. Mewtwo also has better frame data overall in comparison to Ganondorf, on top of having more mobility both on the ground and in the air. And for Smash Bros in particular, recovery also plays an extremely important role, which Mewtwo also has the edge on. And despite how he has weaker KO potential in comparison to Ganondorf, he also has access to a KO throw and KO projectile which are things that Ganondorf lacks. Plus he has a reflector to help deal with opposing projectiles, and a teleport to help with trap situations. Plus, more reliable combos thanks to Dtilt, Utilt, and Nair in particular. And while Flame Choke is more reliable for 50/50's than Confusion, Mewtwo also has access to his jab 1 which can lead into potential KO setups such as grab -> Uthrow or a lucky Disable. Lastly, speaking of Disable, it will probably be far more reliable and useful in an actual match than Warlock Punch would be simply due to being much faster. And unlike Warlock Punch, a Mewtwo player has the ability to choose how to finish his opponent off afterwards via Shadowball/Fsmash/Dsmash/Uthrow/Usmash/etc depending on the situation and location on the stage both players are at.
Oh and having an actual Utilt also really helps too. Though Ganon's Ftilt is much better so there's that to consider as well.
While I feel as though Mewtwo should be a bit heavier, he is most certainly useful and viable competitively. Definitely not the best character in the game, but certainly not the worst character in the game either. Like many people have said, he is a glass cannon. And while he does have his flaws, he has a lot of potential in regards to invoking just as much fear into his opponents as well.
I'm just glad that he is much more viable this time around in comparison to his Melee incarnation.
That, and that he is actually in this game to begin with. That is enough to make me happy.
