That's true, but you do have to remember Hydreigon gets stuff like Flash Cannon for Fairies and other very useful tools like Fire Blast, Iron Tail, Surf, Earth Power, U-Turn, etc. It also has higher speed, which is another problem I have with Hoopa-U is that it's so slow, Hydreigon doesn't hit the 100's but at least it can out run dangerous stuff like Tapu Lele, Hoopa kinda has to run scarf in other to not get revenged killed by basically everything, while Hydreigon can run life orb and not be so punished for it.My biggest problem with hydriegon is that hoopa-u can perform a very simillar role, being able to get the KO's/2KO's in a easier way(beating fini, chansey and clef too), doesn't have to rely on prediction to much since specs dark pulse hits everything fucking hard and while he lacks a good defensive typing he have a nice sp.def that allows you to switch in things like pex and instant treat the enemy, being a switch to a-marowak is awesome i admit, but that means you have to run roost+LO and then you failed to 2KO pex, and i really hate miss a prediction and throw a draco meteor on a metagross. Taunt is deadly agaist pyukumunku, and this is the reason he is only viable in stall, where his teammates can take care of the taunter, and he only competes with clefable, quagsire and shedinja for a teamslot(maybe mantine?) and i think he is at least on quagsire level of viability.
Not arguing the Hydra is better, but it does have an edge over Hoopa in some ways