Hydreigon is awesome in that it can 2HKO everything. Nothing is a safe switch-in to the mixed set. I feel that Hydreigon is overlooked because of Latios, who has notably higher Speed and also a more powerful Draco Meteor. But what sets Hydreigon apart from Latios the most, in my opinion, is its access to Superpower, Fire Blast, and U-turn, as well as the ability to present an incredibly potent mixed threat that checks a boatload of things. That said, Hydreigon's base 98 Speed leaves much to be desired, as it is revenged (or forced out) by too many things that outspeed it. It can work with Tailwind or other forms of team support such as Dual Screens, but due to its disappointing Speed, it's often either revenged by something faster or by something that can take advantage of the move it's locked into, in the case of Scarf being its held item as a "solution" to its Speed issue. A weakness to the priority moves Mach Punch and Ice Shard also bring it down a notch. It's becoming a less used mon in OU because people have had time to realize it can really only get one kill (without support/Tailwind) before it's revenged or forced out, resulting in either a dead Hydreigon or a loss of momentum. I definitely acknowledge what Hydreigon is capable of as an attacker and it has the potential to be great, but I do not find it very viable for the most part. I find Latios and even offensive Latias to be better choices for an OU team.
I would just like to add, if Hydreigon WERE to drop to UU eventually, Scarf Flygon, Scarf Heracross, and Mienshao would have a field day with it. They are already very good UU mons in their own right and having Hydreigon in UU would only bolster their usefulness even more. It can definitely do damage if it manages to get a Tailwind up while the opponent switches to try and save their active Pokemon, but it would by no means overwhelm UU, and many UU teams won't even have to adjust to be able to deal with it.