252+ SpA Hoopa Psyshock vs. 252 HP / 252+ Def Blissey: 289-342 (40.4 - 47.8%) -- 4.7% chance to 2HKO after Stealth Rock and Leftovers recovery
Blissey has to be at 100% and even then it risks a 2HKO from Psyshock.
You're really gonna want to be running timid even with a scarf. There's also the matter of being choice locked into a psychic or ghost type move. The former is asking for a pursuit trapper to come in, and the latter is a free swap in for every normal type. I never said it cannot do work in UU, I'm just saying that scarf is going to have a rough time with all the free swap ins it's going to hand out.