I'm hesitant to assume we'll only be seeing these guys in Ubers. Their stats are distributed weird, making it a lot harder for them to put their full BST to use compared to others with similar totals, and Fairy/Dark/Flying aren't nearly as scary types as ones like Dragon and Water. Given how Kyurem turned out with its own typing issues and very similar stat distribution, it's worth considering how they'd fare if given a similar chance.
Hmm, well Yveltal's good-yet-inconvenient speed tier is somewhat of an issue, and it does have trouble with a lot of common scarfers such as Kyurem-W and Kyogre since it generally prefers the ability to switch moves. It's still a powerhouse though with both offensive stats exceeding Ho-oh's Attack, access to three monstrous STAB Flying moves and an ability that gives a free life orb boost to its STAB Dark Attacks. I still think it is a bit too strong for OU, but I don't see it being as broken as, say, Mewtwo or Kyogre.
Xerneas however would never be allowed to go into OU, or would at least be quick-banned if it was. The already-common Power Herb Geomancy set would single-handedly break OU, and Xerneas' massive bulk, good typing and general power mean it would have no trouble setting up and annihilating the competition; it has more mixed bulk than ANY Pokemon outside Ubers - excepting Cresselia (who sucks) and Multiscale Dragonite (who is probably gonna be knocked down a couple of pegs by the new type).
Even aside from that, it basically single-handedly halts all dragons not called Dialga, even in Ubers; since very few of them are capable of hitting a bulky set even decently hard aside from an Ice Beaming Kyurem-W (who is swiftly blown to pieces by Moonblast), it basically stops half the Ubers metagame on its own - even Soul Dew Latios is useless against it! I don't see anything except maybe Steelix (physical sweeper only) and Registeel halting a sweep from it outside Ubers, and those two suck at pretty much everything else.
Really, the only way Xerneas would be able to be OU would be if lots of really powerful Mega Evos were to be allowed in OU; and I don't see Mega Gengar and Mega Lucario being let loose in OU for very long.