No bias implied, I seriously haven't had troubles with Darkrai. I tested right away, and thought the whole "Heracross" checking thing to be dumb. Whenever somebody sees Heracross and they have a Darkraii it seems they are very hesitant to let out Darkrai, and then when they do, come in and resist the STAB+Focus Blast and then retaliate with a CC. Scarfed obviously and from there Heracross wreaks havoc on everything and Darkraii ceases to be a threat.
I find that since we were so used to Breloom sleeping everything (With a presumably more accurate move) and Breloom having better STAB (For both Metagames fighting is still an amazing STAB) and having access to powerful priority that allow it to surface as a revenge killer, Darkrai putting things to sleep is nothing new. Also with the prevalence of Scarf Terrakion (People finally catching on to how good it is) Darkraii should be weary of setting up because that +1 in attack on top of the Scarfs boost to speed spell a lot of damage.
Overall, I will keep testing to see if Darkrai becomes an immediate problem, but prelimenary testing suggests it is plausible in the current DW metagame (Standard is up for debate, but we still face Base 121 speed Tornadus with stronger moves)