>>17894100As an offensive Pokemon, Yveltal 2HKO's the metagame. Most things slower than it that aren't dedicated walls get 2HKO'd and most offensive Pokemon get 2HKO'd by the appropriate move followed by Sucker Punch. It also still retains the ability to be one of the bulkiest offensive Pokemon and has a surprisingly large amount of longevity with Oblivion Wing.
As a defensive Pokemon, Yveltal can break stall with a simple set of Foul Play, Toxic, Roost, and Taunt. When invested physically, it has greater physical bulk than Skarmory while also having the ability to break stall with Toxic + Taunt and it has the strongest Foul Play in the game. A specially defensive Yveltal functions similarly with slightly different targets.
Overall, Yveltal is a Pokemon that is much better in practice than it initially looks on paper. Simply put, raw wallbreaking power, longevity, and versatility make it an Uber as well as indubitably one of the best Pokemon this generation.