Well of course Hoopa is better than Hydreigon. That isn't much of a debate. However, Hydreigon is a Ground Immunity, is bulkier, and is a Ghost resist which is sorely lacking on every team. Hoopa is none of those things. Hoopa is usually locked into something as well and easy to revenge kill due to that. It also falls over to Ash Gren while Hydreigon resists both STABs. Thing is that Hydreigon has a good niche while Hoopa is just more easily spamable. Hydreigon may not be as good, but it most certainly is not trash.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.
As for Pyuk... The problem is that Clef, Quag, and Shedinja are all better. Clef can wish pass and is a Fairy Type. Quag isn't as passive and can get burns and has EQ, and Shed has Wonder Guard. Pyukumuku can only poison things and BP out. It just has less versatility. :(