Why should Giratina Draco Meteor you when he can just Dragon Tail you out? Giratina outspeeds Scarfty too LOL, so he burns you first if you are using BU. Scarfty isn't amazing at all to be honest, it's weak, not bulky by Uber standards, doesn't have a good typing and many things can set up on Kyogre stuck on Ice Beam. Why won't I use Bulk Up Dialga over Scarfty? Things like Lustrous Orb Palkia and Mewtwo's Aura Sphere 2HKO Scrafty easily.
Darkrai sweeps way better than Scarfty, and I think Scarfty can't even OHKO Darkrai with HJK without investment. Depending on Shed Skin isn't the best option either, give the opponent a free turn and you are dead. At least Lucario can outspeed slow Ubers and has a better typing, more Attack.