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

1.) It is has only been established that Ukitake's shikai can absorb projectile-based attacks. It is likely not possible for him to absorb attacks that are not projectiles such as Katen Kyoukotsu's Kageoni (shadow game). Even certain projectiles are likely unable to be absorbed such as Senbonzakura which is not really a projectile as it is a long range extension of Captain Kuchiki's zanpakuto.

2.) There is likely a limit to just how much or how strong of an attack Captain Ukitake can absorb which is likely based on his own spiritual energy. For instance, too many attacks from Ryujin Jakka would likely wear him and his zanpakuto out and finally result in damage to his zanpakuto. It is also likely that any attack from Zanka no Tachi would be too much to be absorbed.

3.) As per reason 2, Captain Ukitake's health is a large issue. His frail health is likely a large reason for his lack in use of his bankai as use of even a shikai can be demanding on the body when coupled with combat.

Captain Ukitake is certainly strong, but there is not much in theory that would make his bankai one of the strongest. It's properties are actually very limiting as they are situational and very reliant on being paired against an opponent who relies on projectile-based attacks (Starrk, for instance).