I had been able to pick up Nefarian successfully most times in the past. We've had dozens of attempts, and have made it to phase 3. But this time, for some reason, he refused to stay on me at all.
I was taunting (growling at) Nefarian when he landed.
This is the first frustration right there. If I try to taunt him before his feet are on the ground, he ignores the taunt, resets, and goes after the healers while my Growl is on cooldown.
So I try to taunt him the second he lands. He turns to me, and starts attacking me while I start Mauling and Mangling.
Then, he just randomly turns around and kills a healer. While I'm smashing at him. While Growl is on cooldown. While I even had the benefit of Misdirect.
I think success in this encounter is, in general, too dependent on RNG. Threat issues when picking up Nefarian are part of this problem.
EDIT: Apparently this is caused by Levitate and Path of Frost abilities, which in conjunction with the fact that the whole platform is a big elevator, causes the boss to "think" that the tank is in an unattackable position. Will try again canceling Levitate tomorrow.