Honestly not sure why Gengar or Rocky Helmet Ferrothorn would be considered stupid. Gengar is a pretty boss pokemon as it is, with immunity to Ground, Normal, and Fighting moves as well as Toxic and Poison status. It makes an incredible sweeper and has enough utility moves to make a lot of trolly sets. As for RH Ferro, it works fairly well as a countermeasure against Talonflame; between Iron Barbs, Rocky Helmet, and recoil from Flare Blitz used on a fully defensive Ferro, Talonflame (assuming no HP investment) will be losing nearly 2/3s of its HP. If it's coming in on Stealth Rocks, that means it's gonna be sacrificing itself to take out Ferrothorn, and if not, then it's either going to have to Roost next turn or, if it's Banded and is stupid enough to stay in, is easily taken on by a defensive Fire type or anything with Flash Fire.
As far as legendaries, why would they be stupid? They're still subject to the tiering system. And that's also why ALL pokemon have the chance to have 31 IVs in simulated battles. Yeah, Mega Kangaskhan is banned to Ubers, but in Ubers it's allowed to have perfect IVs, just like Keldeo and Landorus are allowed their perfect IVs, and just like they'll be facing opponents with perfect IVs. The fact that EVERYONE is allowed to set their pokemon up that way means everyone WILL set them up that way, unless they have a reason for wanting a different number (such as reducing some to get a particular Hidden Power, or minimizing Attack to minimize Foul Play damage, or reducing Speed to get a stronger Gyro Ball or better speed in Trick Room). The only way that'd be unfair is if Wifi players could face teams created with a simulator, which isn't really possible. Unless you're directly copying your teams into the simulators right down to their actual IVs, which is just plain dumb.
Kangaskhan, though, I agree with. It has no business in OU or UU. Ubers, yes, on account if its Mega evolution's fantastic ability not only making it amazing with Power-Up Punch, but also making its defensive variant have a double-strength Seismic Toss that can even break Blissey's Substitutes. Normal Kangaskhan though? No. Hell no.