User blog comment:Tekkphreke/Theory on Ukitake's Bankai/@comment-7092747-20130913040747/@comment-5551986-20130913223647

If his bankai wasn't one of the strongest, then he wouldn't have warranted a claim that he is one of the strongest and oldest captains in soul society. They have reasons for not using Bankai, presumably for the same reason others have not used their Bankai, it is probably too dangerous. I'm not saying I know what his Bankai is, or agreeing with the blog, simply stating if he had a pathetic Bankai, Yamamoto wouldn't have claimed him and Shunsui to be so strong, despite individual prowess or cooperation.