User blog comment:Bobbs3k/Can Ichigo's New Bankai be stolen?/@comment-10961564-20130630172512/@comment-6657474-20130703022822

Actually, the Quincy impersonating Yhwach had the ability to steal Yamamoto's bankai with his medallion, but was specifically ordered not to by Yhwach. Yhwbach explained that the reason was that only he had enough spiritual power to effectively wield all of Zenka no Taichi's abilities. That being said, until more information comes to light to disprove it, I believe that Skitzo1 and Kwhyman may be correct. Thus far, all the Quincys' usage of stolen bankai has proven to be inferior to that of the true owners'. Thus at this point we can surmise that although Yhwach claims to have completely stolen Zenka no Taichi, his proficiency in using it in the future may prove less potent than that of Yamamoto.