May I inquire as to why Alolan Marowak is in OU? It still has 5 weaknesses, and it's stats are pretty good but nothing great. Is it moves, fairy resistance, or something I'm totally missing?
It's very anti-meta. It's 7 resistances (fire, grass, ice, poison, bug, steel and fairy) and 3 inmunities (normal, fighting and electric) helps it swich against a lot of common stuff like Pheromosa, Koko, Charizard-Y, ect. It also has a signature item which doubles it's attack (Thick Club), effectively giving it a whooping
392 atack with a neutral nature and zero investment whatsoever. Offensive wise it has stuff like Flare Blitz/Fire Punch, Shadow Bone & Bonemerang/EQ, all of which can easily kill something frail on the switch. It also has Stealth Rock, Will o' Wisp and Knock off support wise, so it can do a lot for a team.
Also agree with
jacksoras that Primarina needs to rise a rank or two. Water/Fairy it's a pretty neat typing with nice coverage and one extra resistance and inmunity (dark and dragon), giving it some chances for switches (mostly on Specs Greninja, though it can also work against Lati@s, Keldeo and others). Prima's movepool gives it quite a lot to work with; it has Psychic for M-Venusaur and Amoongus, HP Fire for Ferrothorn and Scizor, Energy Ball for bulky waters, etc. For water STABS it can use either Sparkling Aria, which bypasses substitutes while also healing burns, or Hydro Pump. Torrent surprisingly works well with both of them, giving it an insane boost when it's low on HP, effectively letting it hit stuff really hard before dying (which happens a lot).
How effective Prima is in the long run depends of the type of matchups. It works pretty well in a fire/grass/water core (stuff like Amoongus, Venusaur, Heatran and A-Marowak are nice partners) and steels, and can do a lot against HO that lack Koko and TR teams (the latter which are starting to be a thing again), but it struggles a lot against bulky offense, specially when the special walls are still healthy.
It IS by no means the go-to-pokemon for wallbreaking shenanigans, though. Base 60 speed can be pretty underwhelming, strong physical STABs are potentially dangerous, there's quite a lot of special walls running around (which is due to Lele's popularity) AND it faces a lot of competition from Lele (worse bulk, but it's faster and has a better ability), Azumarril (better physical bulk and can use Belly Drum), and Volcanion (Slightly faster and resist Fairy, but has bad special bulk) for that slot.
Some random calcs:
252+ SpA Choice Specs Primarina Hidden Power Fire vs. 252 HP / 168 SpD Ferrothorn: 324-384 (92 - 109%)
252+ SpA Choice Specs Primarina Psychic vs. 248 HP / 92 SpD Amoonguss: 340-402 (78.8 - 93.2%) -- 37.5% chance to OHKO after Stealth Rock
252+ SpA Choice Specs Primarina Hidden Power Fire vs. 248 HP / 200 SpD Scizor-Mega: 352-416 (102.6 - 121.2%) -- guaranteed OHKO
252+ SpA Choice Specs Primarina Sparkling Aria vs. +1 0 HP / 4 SpD Volcarona: 306-360 (98.3 - 115.7%) -- guaranteed OHKO after Stealth Rock
252+ SpA Choice Specs Primarina Psychic vs. 252 HP / 0 SpD Toxapex: 234-276 (76.9 - 90.7%) -- 25% chance to OHKO after Stealth Rock
252+ SpA Choice Specs Primarina Psychic vs. 248 HP / 144 SpD Venusaur: 274-324 (75.4 - 89.2%) -- 12.5% chance to OHKO after Stealth Rock (watch out for this one)
One last thing:
(...) can we please stop comparing Primarina to Fini, they don't do remotely the same thing. Compare Primarina to Lele if anything.
EDIT: Changed and added some extra info here and there.
EDIT2:
Here's a replay of me playing badly and somehow Metagross and Primarina saving the day. Would've done more if it had Hydro Pump instead, but eh.
(In hindsight that probably wasn't the best replay)
EDIT3: Another one. Mostly Primarina forcing switches and walling Specs Greninja like it was no thing.