hydrei's at a perfect spot rn honestly
it still has its good matchups against specific things, it's one of the few true effective offensive mons with tailwind, and the combination of speed + resistances still serve as really handy. It's a mon that's capable of aiding the mu against 3/5 in t1 while in t2, it takes whatever tran and hoopa can do, can smash aegi, and can chunk a good amount on scouted-as-non-scarf lele. Further down the list, there's still specific favorable-to-fine mus against a varied pool of threats (MGar, Zygarde, Zong, Zap) which give it a high probability to always be a useful in-game piece that can keep certain foes in line. Also, the fact that it has a lot of utility for an offensive mon is pretty sick as the resistances and threat of tailwind can make foes respect it enough in-game to adjust a substantial amount. In addition, the threat of either getting hit w/ a strong attack or having twind set on your face is a specific thing that it has that can often be a game changer, only seen in the mega-costing Mega Salamence and (more uncommonly) Mega Charizard Y. (a variant of this conundrum, w/ tr, can be seen on hoopa-u)
it has as much unfavorable mus, however, starting with Mega Mence and the Tapu meta. Getting owned by really common stuff (and wholly destroyed by Fini, thanks to Misty Terrain) is a huge pita to deal with. For an offensive pokemon, its speed is dragging it down as it doesn't have the luxury of high bulk that allows it to comfortably tank through faster offensive foes, and its attacks, when it's not dracoing, don't really feel that they do enough in this bulky-ish metagame; being flawed as an attacker is a bit of a drag to its effectiveness, the only reason it's scraping by is matchups and tailwind, really.
putting hydrei in either 4 or UR is absurd, it does too much to be relegated down there. If it's ever moving a spot, it's up to 2, though i personally don't have any opinion on that one atm.