I'm not huge into competitive pokemon, but how did Game Freak manage to make a 'mon with 194 Special Attack and 140 speed irrelevant?
Basically, it boils down to it being a Mega Evolution, thus using up a Mega Evolution slot. AFAIK, in Smogon's Ubers (basically close to a free for all singles format except Mega Rayquaza is banned), it isn't totally hopless, since it has a niche of being a Psychic Terrain sweeper that has Fire Blast and decent bulk, but it competes heavily with Deoxys-Attack and Ultra Necrozma, two mons that don't require Mega Evolution, while Mega Mewtwo X has a big problem with Fairy-types. Besides competition, the Mewtwos also offer almost no defensive capabilities worth talking about since gen 6 (believe it or not, defensive Mewtwo used to be a thing).
Now that's just for a very popular fan format, where Mewtwo still has a niche. In Gamefreak's own formats where they have allowed Mewtwo, the news is much more grim. Mewtwo preforms pretty poorly in Gamefreak's current VGC format. Looking at its usage now... its currently #36 in the Ultra Moon VGC format, which is bad news considering how centralized that format is. In general, Mewtwo struggles with doubles formats because not only does it not have spectactular bulk for a doubles Pokemon, but its got no good multitarget moves to speak of, and its weak to popular things like Incineroar and Lunala (before you mention Shadow Ball, Shadow Shield ensures Lunala will survive that, while Mewtwo sure as hell is not surviving Moongeist Beam lol). When Ultra Cup comes about and allows Megas, I expect people to crowd around Mega Gengar, Mega Salamence, and Mega Rayquaza over the Mega Mewtwos for similar reasons.
Even in a singles format for Uber mons Gamefreak made, for two Special Seasons where you're allowed to use one Uber, Mewtwo was considered a pretty iffy pick, when you could use a much more multipurposed Pokemon such as Ultra Necrozma or Mega Rayquaza instead.
Pretty much the only particularly bright spot for Mewtwo this gen is Let's Go, where its a fucking monster. That sorta illustrates Mewtwo's biggest issue: in a vacuume, its really strong, but even with its mega evos, it just hasn't adapted to its sorunding envoironment enough.
Fresnel rim light, bloom is something different
If they cared about balance, they wouldn't have let the two forms mewtwo already has get outclassed
To temper your usual pessimissim for a bit, when the Mega Mewtwos had their stats found out in XY, people WERE pretty terrified of them for much of XY's competitive life. Because yeah, on paper, their stats are nuts. It was more of a slow realization that raw stats isn't what Mewtwo needed, it needs a new typing and a more suitable ability for its offensive focus. As far as I rememember, Gamefreak balanced gen 6 doing internal tournaments related to Battle Spot Singles and Battle Spot Doubles (there's some interviews on this somewhere I believe, I remember them posting some results of a doubles tournament as an example where Talonflame topped their usage). So yeah, Mewtwo was probably not in the plans for balancing, so like many of us at the start of XY, I bet Gamefreak just assumed giving Mewtwo a raw power increase was good enough.
Interestingly enough, maybe due to VGC 16's boxart Uber focus, I feel that gen 7's boxart legendaries have had some very deliberitlatey focused stat spreads. For example, Solgaleo and Lunala are 1 point slower than Yveltal, one of their biggest and most important checks. Meanwhile, Ultra Necrozma is 1 point slower than Mega Gengar, a MAJOR mega it will face in VGC 19, especially since Mega Gengar gives all Necrozma formes big issues. So I wouldn't be surprised if Gamefreak starts balance testing these mons with a doubles focus on mind.
Even the mighty Mega Rayquaza, whose completely busted in 6v6 singles formats we like to make, actually has some strong checks in Gamefreak's format. For example, in Gamefreak's singles Special for Ubers, Mimikyu was such a consistent problem for Mega Rayquaza that Fling Kings Rock Mega Rayquaza became a standard set, which lowered its offensive power quite a bit! As for VGC, while Mega Rayquaza has shown itself to be a powerful enough mon to muscle through similar issues Mewtwo has faced, I'd argue the main reason it works out is because its an incredible partner for Primal Kyogre to get rid of Desolate Land, so it can snipe Primal Groudon.
