A Smiter is not the only source of stun. With a few more levels to my Singing Barb and the last few items I want for him, I will be able to cast a 7+ second stun, and he already has enough fcr to hit the 8 frames per second cast rate. Even if the Ubers reduce stun length substantially, if the stun will last for longer than 8 frames I cause a stun lock (or I will be able to). Also, my barb currently has a level 33 BO on weapon switch.

But can the Ubers be stunned? Their act boss counterparts cannot be stunned (doesn't mean they can't, I'm just saying).