It's certainly true that Hydreigon has been hurt by the introduction of the Fairy type; however, I'm skeptical that a 4X weakness to Fairies is any worse than, say, Garchomp's 4X weakness to Ice. There may be more competitively-viable Faeries than Ice types, but I suspect that Ice is more popular as a coverage type than Fairy is, so Ice attacks are probably more common that Fairy attacks in the metagame. And as mentioned earlier in this thread, Hydreigon got some buffs of its own, as Dark Pulse is now more spammable and its Dark typing is one of only two types that either resists or is immune to Ghost attacks.
I use a Choice Scarf Hydreigon as a teammate to AV Metagross, and it works quite well. Hydreigon resists or is immune to all of Metagross' weaknesses aside from Mold Breaker ground attacks, and Metagross covers Hydreigon's Ice, Dragon, and Fairy weaknesses. Aegislash would likely be an even better partner, as it resists Bug and is immune to Fighting attacks. You should probably add a third teammate who can more easily handle Fighting-types with Dark-type coverage moves, though (Conkeldurr I'm looking at you).
Hydreigon still wishes it had just a little more speed, and its Dark typing gives it at least as many drawbacks as advantages, but I think it's still very viable in OU.