Firstly, I question how a Pokémon like that could be labeled as having a limited movepool. It already boasts access to the highly spammable Dark Pulse, can utilize Sludge Bomb against Fairy types, and has the option of running Focus Blast and Bolt Beam while doubling its Special Attack twice. This alone highlights its potential drawbacks on the tier.
As
PapaMax pointed out, the inclusion of Knock Off in its moveset is incredibly powerful, shutting down Pokémon that you consider long-term checks. With these versatile options, Darkrai can easily select its checks without sacrificing much, not to mention its array of support moves. Simply opting for Z-Dark Pulse or Z-Thunder/Thunderbolt provides ample firepower to overcome numerous obstacles.
As for the bulk argument, darkrai is bulky enough to be dangerous and not just a glass cannon. It is bulkier than greninja (while being faster) and tapu Koko, and those mons are not particularly easy to revenge kill. In the case of gren or Koko, you either got a scarf/a naturally faster mon (which is limited to zam, punny, Koko mainly since you need to activate battle bond for gren so I don’t consider it) or a wall to get rid of them. But for darkrai, how is it sustainable when the only way is to revenge kill it with faster mons, since its coverage just get rid of the wall possibility lmao?!
As for the priority argument:
252 SpA Choice Specs Greninja Water Shuriken (15 BP) (3 hits) vs. 4 HP / 0 SpD Darkrai: 102-126 (36.1 - 44.6%) -- approx. 3HKO
0+ Atk Technician Scizor-Mega Bullet Punch vs. 4 HP / 0 Def Darkrai: 111-132 (39.3 - 46.8%) -- guaranteed 3HKO
Also just to come back to this, power creep is much more important in Gen9 and can’t be compared to even Gen7 with just overall extraordinary new Pokemon that can compete with it.
So I think Darkrai would not be a healthy addition to the tier due to how it can overwhelm what would be answers and how it would wrap teambuilding.
If there was a change to do to the tier, it would be to ban volcarona