Board Thread:Questions and Answers/@comment-10797017-20130627100621/@comment-8986886-20131010113408

Kyūkyoku KON wrote: As a side note, in addition to finding a way to get back stolen Bankai, I think it answers why Ichigo's Bankai couldn't be stolen and makes me suspect that none of the visored shinigami who possess Bankai could've had theirs stolen as well.

I'm not sure it explains anything, and let me explain why. Asguiaro Ebern was the first to try and steal Ichigo's Bankai, using the Quincy Medallion (so Ginjo doesn't count) If Ebern had been a Quincy he would never have tried to do so, because the residual Hollow traces within Ichigo would have put his very existance in jeopardy. Perhaps Yhwach had planned to have Ebern steal Ichigo's Bankai, and because he was an Arrancar it would be safe for him to do so. Considering that, Ebern failed because he wasn't strong enough to steal Ichigo's Bankai.

Quilge Opie, however, contradicts this theory, since he was a Quincy and he still tried to take Ichigo's Bankai. Furthermore, Quilge used his Vollständig to absorb Ayon, a beast created using the sacrificed arms of the Tres Bestias, which in retrospect should have killed him.

My concusion is that I'm not sure Kubo thought this through properly, because these events seem to contradict each other, and with what we now know from Kisuke's explanation.