When discussing mons and how balanced/broken they are, I think it's always a good idea to begin with its best set, and use that as the foundation to build further arguments.
I COULD BE BANNED (Ursaluna-Bloodmoon) @ Grassy Seed
Ability: Mind's Eye
Tera Type: Poison
EVs: 248 HP / 128 Def / 76 SpA / 28 SpD / 28 Spe
Modest Nature
IVs: 0 Atk
- Calm Mind
- Blood Moon
- Earth Power
- Moonlight
So listen I think the evs are uh...flexible, but I'll explain mine. 76 SpA modest ensures that BM-->EP-->BM kills sdef dondozo even if they are lefties, 28 SpD ensures you live one focus blast from gholdengo if needed, 28 Speed is used to outpace blissey, then simply max HP rest in defense.
This pokemon, as we've seen, forces unreasonable and subpar sets in order to be dealt with adequately. After a single calm mind and grassy seed proc, it is a hulking monster with 429 hp/452 Def/535 SpA/259 SpD and is insanely difficult to take down. I'm going to walk through the three best answers to this bastard bear, because the others are quite frankly shit.
1) Corv: Sub+bulk up+roost+brave bird corviknight with max sdef is one of the best answers you can manage. You can safely roost behind a sub, and with enough bulk ups, brave bird can hurt ursabm even if it is tera poison. The problem is that this set is a passive piece of shit. It is incredibly easy to take advantage of a corviknight whose only attack is brave bird and cannot even u-turn. It is free real estate for sd kingambit, heatran, zapdos, rotom-w, etc all mons which you could've body pressed or at least u-turned on.
2) Gliscor: The other solid answer is sub+protect+eq+toxic gliscor. Pretty self explanatory what it does, u keep up a sub bc blood moon can't be used twice in a row then click toxic/eq as needed. The problem with this set, funnily enough, is the interactions with opposing gliscor. When you fight a good gliscor set (eq/spikes/toxic/protect), you are letting that shit in for free and letting max layers go all the way up. If you run into SD gliscor, that's also setting up for completely free on you. The worst part here is that you're gonna hard lose if ursabm ever decides to go hyper voice, which is a fairly common option.
3) Cresselia: A standard cress set actually does well, no footnotes. Obviously this isn't splashable counterplay though, and must suffer through the very common kingambit/encore usage in this tier. If you want to set up on gliscor, you must burn your tera to poison/steel, meaning you can't go tera blast fight for kingambit like you could in the home meta. With Encore being a strong option on every ogerpon and gliscor forcing you to tera, it's safe to say cress got worse from home to dlc.
I am ignoring shit like cm psyshock bronzong, tera ghost air balloon NG Gweezing, and other unviable gimmicks. I will also ignore counterplay that involves tera like Sub tera grass iron moth or tera grass protect leftovers heatran, because requiring a tera use to check a pokemon which hasn't used/doesn't need to use its tera is a very bad idea. Using tera to cover a hole in your teambuilding still leaves you at a significant disadvantage, because you have used your tera and your opponent is free to use theirs without fear.
Some argue that forcing ursabm to tera poison is great! So what if it turns your gliscor into set up fodder, you can still deal with it! The problem is that Minds eye Blood Moon+EP is damn near perfect coverage and no single tera type will save you defensively besides tera steel+levitate mons. So using your own tera to handle ursabm's tera is rarely an option (and even when it is, something like tera steel rotom-w is not breaking a cm tera poison ursabm). Your tera has to be offensive and punch through its massive bulk somehow, this rarely happens. In general, the better a pokemon is, the harder it will abuse tera. Not all teras are made equal, and this one is much more overwhelming than what most of OU could do.
The above counterplay is either insufficient, subpar, or not splashable in the slightest. This pokemon is absurdly strong and bulky. I will be voting BAN.