[Lord of Bays]

Well, every Justified Pokémon is risking taking serious damage or SE coverage in the face upon switching in. Draco Meteor is going to hurt EVERYTHING, resistances be damned, Fire Blast clocks Virizion, Cobalion, and Lucario, and Superpower/Focus Blast/Earthquake/Earth Power crushes Cobalion, Terrakion, and Lucario. The only thing that Hydreigon doesn't cover is Keldeo and Keldeo can't switch in on anything but Superpower or Earth Power. Everything else (Draco Meteor, Dragon Pulse, and Focus Blast) do upwards of 60% with a Life Orb and positive nature, with Draco OHKOing outright.

Anyway, Dark Pulse is really only useful for SpDef Jellicent. Practically everything else that's common in OU and that is weak to Dark is covered in some way by a different move or Hydreigon can't do anything if they get in safely. Espeon, Gengar, Starmie, and Alakazam are way too frail to switch in to anything but Focus Blast or Superpower (or a Ground-type move in Gengar's case), but once they're in they can force Hydreigon out with Fighting-coverage; Lati@s is in a very similar boat, only they can't switch in on Hydreigon's Dragon STAB and can then threaten him out with their own Dragon-type attacks; Reuniclus and Deo-D are bulky enough to weather a hit (though probably not well enough to switch-in on an unresisted hit) and retaliate or set up; and Celebi is hit by Fire Blast, so that's a moot point.