[eplipswich]

I doubt Aizen's group is actually underpowered. Like what Question said, Aizen's shikai ability can easily defeat all the people in Soul Society.
Aizen is, after all, the ultimate boss, that's why no one has yet figured out a way to even touch Aizen. (Well actually Aizen's shikai can be countered, but only if he's up against a blind shinigami like Tousen (too bad Tousen is a villain...and actually i doubt Tousen can even beat Aizen even if he's blind o_O, but someone who can master the art of blindness and is stronger or as strong as Aizen can probably defeat him)]

Aizen's shikai is not 100% perfect. For example, when Aizen faked a corpse of himself, the 4th squad captain could detect that something was amiss with Aizen's fake corpse, but well...she wasn't strong enough to overcome his shikai.

The thing is it's still a very long way before Aizen is even a match for Ichigo.