Board Thread:Theories and Speculation/@comment-1872728-20151008094957/@comment-4646462-20151008214119

Asura probably had Wood Release as well as the Yang chakra that the SPSM/KCM is composed of. It's possible that pre-Hashirama reincarnations had some degree of either or (or even both), but it's likely that Hashirama and Naruto are the strongest reincarnations, with previous ones showing power that is not particularly godlike or having died before they could use their power to its fullest extent.