Fml i completely forgot about the Speed boost so don't mind me. Focus Blast / Earth Power is going to anihilate those two so yeah, nothing can save us from Xerneas.
I think it's going to be a really excellent cleaner, but it will be like Extremekiller in that the metagame will adjust so you can't just bring it in after something dies, use your set-up move and sweep. Compared to Arceus, it:
+doesn't have to worry as much about Scarfers that resist its STAB revenging it, especially on the Special side
+is probably slightly more powerful off the bat and at +2 (I say probably due to the increased special bulk most Ubers have and not knowing the boost Fairy Aura gives)
+much better STAB (Poison/Fire resists rather than Rock resist and Ghost immunity)
? the two coverage moves could be better/worse depending on its actual movepool. having to depend on Focus Blast to hit Dialga or Thunder for Ho-oh (less likely to have Rain up thanks to the weather nerf) could make you a sad panda. It's less powerful than Dialga passed a Shell Smash, and there were still random things that could survive its attacks and phaze/KO/bring into priority range.
-is likely more dependent on its item. If you phaze it while/after it sets up, it probably can't boost for the rest of the match. You don't have to worry about it shrugging off your status move with Lum or KOing with a move boosted by Expert Belt/Life Orb/etc
-less speed off the bat/no priority.
I do think Xerneas will want to use most of its Speed EVs, especially if it's closer to 90-95 Speed than 101. I can envision a scenario where Xerneas wants to outspeed something like Kyogre because taking a hit at +2 Special Defense rather than +0 will allow it to survive subsequent priority attacks. Also, I think stuff like Ho-oh and Sp. Def Kyogre will survive a boosted hit since Thunder's BP is now 110.