So did GameFreak royally screw Yveltal? Xerneas trumps him (Dark being both weak to fairy and 1/2x effective against it) and Zygarde does as well (his ability reverses the effect of Dark Aura). Xerneas's Fairy Aura also gets reversed, but fairy is supereffective against Dragon anyways so it just puts them Xerneas and Zygarde on equal ground.

This coupled with Mewtwo and Charizard's X forms suggest that GameFreak has an inexplicable hatred for the letter Y.