I have issues with Mewtwo. It's already super powerful as it is, and it's available to catch in every generation except for Gen 5. Why, of all things, did it need a Mega Evolution, and 2 of them for that matter? Just why?
I also have a problem with the weather trio and their Mega Evolutions/Primal Reversions. While Groudon's is admittedly good for countering its water weakness, why do Legendaries even need Mega Evolutions/Primal Reversions in the first place? They're supposed to be powerful as they are. Instead of adding Mega Evolutions to legendaries and already overpowered Pokemon, why not give them to Pokemon who don't get any love-like Dunsparce or Qwilfish? Mega Evolutions greatly helped Mawile and (at least to some extent) Sableye become decent competitively.
TL;DR: I have problems with the Mega Evolutions given to Pokemon who could already hold their own in the competitive scene and legendaries, especially Mewtwo.