Anyway now that I have actual experience, here's my opinion. Note there is over preparedness bias.
Darkrai feels like just another special sweeper. Despite a little above average defenses, it's spread of resistances is rather concentrated and is hit neutral by a lot more relevant attacks. Second, it's weak to Fighting and Bug, two rather common attacking types. In the end, Darkrai died pretty easily in all my matches and didn't do much. On Dark Void, I don't feel it's something game breaking. It doesn't have 100% accuracy like Spore, and with it Darkrai's coverage is limited to Dark Pulse and an inaccurate Focus Blast. I can't say anything much about Choice sets. Anyway that's all I glanced from the few battles I used Darkrai in, ggs once again.