Excusing the primals and mega Ray who get 100 extra BP for free, Xerneas is the single most powerful cover legendary ever created. Gamefreak probably wanted to dial the power creep back a little.
Lunala looks pretty solid, I feel. Defensively, that typing is pretty atrocious. It should be offensively too, but Moongeist Beam or whatever it's called could be amazing. Ghost STAB is great offensively and mostly inhibited by the highest BP move being Shadow Ball. If this move is 100 BP or so, it's gonna rock. Stat distribution and movepool are going to determine a lot, imo. If it can join Lugia in escaping the base 90-100 speed quagmire cover legendaries have been wading in and get something like Roost or Recover (seems reasonable) I think it'll be solid. Sincerely doubting we have another Kyurem situation on our hands.
Someone noticed that all the Pokemon featured in this trailer all had 0 IVs, just like how most ORAS screenshots had 100 HP EVs with 31 IVs. This means that Solgaleo's HP is likely base 152(!!!). If Lunala gets that, the only thing stopping it is Yveltal.