Actually its likely that he is. If one can destroy soul society (different from seireitei as Seireitei is only a huge city in soul society), one can destroy the world. Soul Society is the parralel world of the real meaning it should be about the same size. But even if it isnt, the destruction of Soul Society brings about the destruction of the physical earth as well. Based on what Unohana said, continuous use of Zanka no Tachi could destroy the world. So that probably either means he is a planetary life wiper or a gradual planet buster. Best case scenario is that he can obliterate it through sheer power. Either way, he is probably the most destructive character in the top ten ongoing shonen right now (the power 6, Toriko, Beelzebub, Fairy Tail, Claymore and the like). I thought this might happen. Before its end Bleach pulls out some planetary threats of which Yamamoto's bankai was certainly one of them. Juha and Ichigo before the end are probably going to be planet busters or life wipers as well.