I've always found RP Genesect to be its most threatening set. Scarf is ok, but the letdown comes mainly from that base 99 speed. I personally prefer faster scarfers such as Terrakion or Keldeo, or Mence even. While it is nice having STAB on its U-turn, the failure to outspeed things such as +1 mence can be a big let down.But the real problem lies in how easily RP gene can sweep teams, you cannot know his full moveset for certain straight away as some might run Giga Drain, and if one chooses to use an expert belt it makes setting up even easier. The sad thing is when you see people lowering Def IVS on pokemon just to avoid genesect getting a download boost, which is pretty stupid. Things like Garchomp have good bulk that is being ruined due to this iv reducing. Furthermore, heatran now having to run shedshell, which isn't even that great of a set as Heatran really needs lefties. Even the usage statistics are insane, 50 per cent. It's hard not to say that metagame is overcentralised around this bug.
And it can be quite hard to know Genesects set, one move shown in its arsenal does not reveal what set it is. U-turn is common on most sets apart from RP, coverage moves are common through out all sets as well. By the time you find out which set it is it will be too late and your team will be damaged.