IDK, it doesn't seem that bad to me.
Yveltal's Base Stats:
HP: 126
Attack: 131
Defense: 95
Special Attack: 131
Special Defense: 98
Speed: 99
BST: 680
That BST is on par with many other powerful legendaries. Admittedly base 99 Speed is terribly awkward, and you'd think that a Pokemon that is the embodiment of death and destruction would have offensive stats similar to that of the Mega Mewtwos or something, but then you have Talonflame, which only has base 81 attack but hits like a truck.
Yveltal's got some other things working in its favor. While Dark/Flying gives you weaknesses to 4 common attacking types (Rock, Electric, Ice, and Fairy), it still resists Ghost and Dark, which is great since so little resists those 2 types in 6th Gen. It's actually a pretty good typing offensively. Dark and Flying are really spammable attack types since there's not too much to resist either one, and the only Pokemon I can think of right now that resists both those types is like Bisharp. Flying also hits Fighting super-effectively, which resists Dark.
Dark Aura is also really helpful to it, giving it a boost to Dark attacks
in addition to STAB. Yveltal's Dark Pulses and Sucker Punches hit pretty hard, and having a priority move is very helpful to it too. Oblivion Wing sucks out HUGE amounts of health, letting Yveltal stay alive longer while doing damage at the same time. Ground immunity is nice too.
It's far from being the greatest thing ever, but it's still pretty good imo.