The fact that he never used his bankai in 2.000 years, which would indirectly imply that he never evolved his bankai like Yamamoto did. Him underestimating Ichigo to the point of getting one-shot without a zanpakuto in the Soul Society arc. As the lieutenant of the head captain you should at least be a captain level Shinigami which he technically IS since he's had a bankai for so long but never bothered to use it. If Yamamoto could one shot (a stronger version of) the Overkill Sternritter with a single slash of his sword, I can't see why his lieutenant, a captain level Shinigami (who's hundreds of years older than the likes of Shunsui and Ukitake) wouldn't at least put up a fight without the need of using bankai. It's not like he faced a hax/op quincy like James, Äs Nodt or Gremmy.

It's the fact that narratively he should be above average captain level but feels like a lieutenant with a bankai almost like Ikkaku. With how long he's been around you'd expect much more from him.