On top of that, the Barrier of Unwill is, at best, inconsistently applied. Witness the Witch King's attack - when pretty much everyone on the battlefield just can't move once he gets his will up and running, despite no explicit lightening of the barrier. But that's pretty much the best mind-domination feat in the stories, and it's LAUGHINGLY short of what Xavier, Cable, Nate Grey, Moondragon, et. al. could pull off, AND none of the Valar actually, you know, resisted it. I'm not saying that they couldn't, jsut that they never did.
Gollum was broken by pure torture, not by any kind of mind control or telepathy. Now, this could mean that Sauron couldn't crack him, but it could also mean that Sauron just didn't bother trying and left the action to his minions - something he consistently does unless he has no other choice in the matter throughout his entire history. He's a plotter, schemer and manipulator. Now, you could chalk that up to a mighty mind-twister being thwarted by the Barrier of Unwill, or you could just say that Sauron doesn't have any kind of powerful telepathy and that "the barrier of unwill" is just normal willpower being used against really crappy low-end telepathy. That's what the feats say, in any case.
Also, the Palentirs kind of explicitely bypass a mind's defenses. This let Sauron casually dominate Saruman, whose mind, at least, should be roughly on Sauron's level, given that Curumo was also a maiar peer of Sauron's before getting squeezed into an Istari body.