The Darkrai conversation is always so weird to me because it seems to be in between Hypnosis and Nasty Plot, and ignoring the rather practical movepool it has. Forget Hypnosis, it has Taunt, Will O Wisp, Trick, and Thunder Wave, which seem like wild card tools to cripple would be checks or walls. It attack options is alright, with options to deal with dark, fighting and fairies who would otherwise resist its STAB, oh and Ice Beam if you just want a good coverage move. Its basically faster than the entire unboosted meta other than Dragapult and Jolly Zamazenta (Timid Darkrai beats Neutral Speed nature on Zamazenta!). And 90 attack makes me wonder if a few mixed sets could beat Dragapult with sucker punch. Specs, Stallbreaker, Wallbreaker, Scarf, Nasty Plot, AoA, and I wouldn't be surprised if a Calm Mind set sneaks in. Darkrai can't do this stuff in Ubers cause of the power levels, but in OU, it's much less one dimensional.
Add in Tera to the mix and it's going to be just another unwallable wallbreaker who can sweep and clean any teams without a +1 speed options left (Or Dragapult or Zamazenta) with a lot of options to do it, and people are still dwelling on Hypnosis being the crux it's viability. There might be an OU meta that can deal with a Darkrai in it, but it's not this one yet.