Zander

It's "Pathetic" to be one of, if not the best, mon in the second highest tier? And didn't you say to not look directly at stats, yet you mention that it's "pathetic" to be in the same tier as Seis (which is a really good mon for multiple reasons, not just stats) while having base 600... Not many Powerful mons in UU? A majority of Gen 7 UU is Gen 6 OU, so that is really inaccurate. Hell, if you measure Pokémon just for power instead of Synergy with teams, support and everything you should probably start from the bottom instead of Talking about how a mon needs a broken ability before it's actually good. Drei needs about as many buffs as Salamence and Rayquaza needed their mega's, not at all. If you still can't see this I legit suggest you ask around in the UU room on showdown how good it is, because at least 5 or 6 people have already told you you are mistaken. And I'd say those people are, in fact, correct

Metagross is also UU. Along with other psuedos just like Hydreigon, like Kommo-o, or UBs like Stakataka. Or powerful Mega Pokemon like Aerodactyl, Beedrill, and Altaria. Gengar is also UU. Metagross actually isn't that great in OU at the moment, as there are plenty of things that deal with it, same as Hydreigon. 

The tiers are based on usage. Sure stuff can work in higher tiers, but not always.

If you think that BST matters, well thats just silly. Goodra is another pseudo and it is in RU (even lower than UU). Archeops has a BST of 567 and is just barely out of PU (the lowest official tier).

I think everyone here pretty much told you why it wouldn't be a good idea to give Hydreigon that ability from a gameplay perspective but don't forget the lore behind this pokémon aswell. Hydreigon is a  highly destructive creature, not a defensive, nigh untouchable wall.

The only abilities I could see being given to Hydreigon lorewise are either Glutonny, which could make a gimmicky Belch set somewhat viable, or since Hydreigon is the Brutal Pokémon, Sheer Force, which could break the thing  thanks to Hydreigon's colorful movepool.