Hydreigon looks pretty nice yes, those purple flaps look somewhat odd. It's good, not a favorite. Dragonite is still somewhat unique for being the only ''nice'' looking pseudo until Goodra appeared. Nowadays I see the point of his Barney problem, and Dragonair looks a lot sleeker, but still, it does it's job. Iris' Dragonite from the anime also helped his image a bit(by being grumpy).
Dragonite has less effective raw stats and typing, however, he does have one of the best defensive abilities, which make him set up very easily, and it suddenly makes it's stats work far better then before, as it can use all that bulk and it's high attack effectively compared to Dragonite-SWEEPERREVISION, AKA Salamence. They aren't that easily compared in terms of roles, Hydreigon is a more special/mixed sweeper, while Dragonite is a bulky sweeper. Draggy has a ton of support moves too.
Hydreigon used to be pretty decent, but his fairy problem makes him objectively worse then Dragonite now. They have the special defense to tank his best attacks, and then he's hit with 4X effective fairy attacks, with no way to reduce it like Dragonite has.In Smogon it actually dropped in tiers as a result. .. And I don't even play competive(I'm to insecurish/have issues with pokemon's ingame with how to prepare for it)