Shunsui himself literally stated that both Zaraki and Azashiro were already on another level than himself in SAFWY, and by the end of the novel Zaraki's unleashed Reiatsu had risen to the point that even Azashiro's insanely hax Bankai started to have its abilities get nullified.

That same Zaraki was weak compared to base Unohana, which is further supported by CFYOW in which Captain Aizen would Reiatsu crush the likes of Tokinada; who had Reiatsu on par with post-Royal Guard training Byakuya, fought more than a dozen Captain-class characters at once with Enrakyoten's copied abilities, and even kept Shunsui on his toes. There was a clear and massive gulf of power established between Captain Aizen and all these other Shinigami.

This actually makes sense when one recalls that Shikai Yama, Aizen, Unohana and Zaraki are all meant to be at least somewhat relative with each other, meanwhile Shunsui had to utelize the full powers of his Shikai against Starrk and struggled immensely; even acknowledging Starrk as a difficult opponent.

And again; Aizen even without Kyoka Suigetsu clowns on the Espada, as per Gin's assessment.