Xerneas is super powerful. Everyone plays Pokémon X. Yveltal is lame compared to the fairy type legend. And even Yveltal’s typing is lame. But it’s got good moves, good special attack and good recovery.
So why does it feel so underrated?
Well, every one is different. Look at this example.
EXAMPLE: I really like Pikachu, but my little sister likes Eevee better.
It's kinda like with Xerneas and Yveltal. Some people like a Fairy (Xerneas) type better than a Dark/Flying (Yveltal) type.
It mainly just depends on the person your asking, Because I like both Xerneas and Yveltal equally.
Did that answer your question? If not let me know. ^-^(I was only going off of what you said and my own experience)