Why is Sosuke Aizen's Bankai never mentioned as a potential threat or even mentioned at all?

By Sosuke Aizen having been a Captain of Gotei 13, he has achieved Bankai, but everyone is threatened by (or useless against) by his Shikai... a Shikai, not a Bankai! Why is that? and what can his Bankai be capable of when his Shikai is so powerful?
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