Let's see now... Plasma Fists has 95 base power I think, which is very good. If it gets just one turn free, it can use Hone Claws for a considerate damage boost, at which point it can hit very hard. It already outspeeds mega Metagross and the common Ubers mega Metagross doesn't even have EQ while getting 2-hit KOed with a supereffective move. If Zeraora learns EQ too (TM), then it can hit Primal Groudon too if needed (although you will want to switch out). Also, what DOESN'T get walled by Primal Groudon?
Mega Salamence will need to set up against something that can potentially have Outrage, which isn't something a lot of people will try. Arceus-Ground and Tapu Lele are pretty much the only things that can effectively deal with it. Deoxys-A can too, but then it has the Psycho Boost stat drop to worry about and Zeraora may even be able to have Focus Sash to avoid a major hit. Not saying it's broken and deserves to go almost to AG, just saying it's too strong for OU. MANY things are weaker in Ubers if you think about it! Also, just because 3-4 mons can deal with it out of the entire tier, doesn't mean it's bad. How many things don't have trouble with at least one of Primal Groudon, Deoxys-A or Tapu Lele? Like half the tier gets destroyed by them.
A fair bit, just make sure you are done with the story first though or they won't show up!