Board Thread:Naruto Discussions/@comment-30323490-20170604171211/@comment-734582-20170605131751

Hagoromo so far displayed a level of power Madara didn't even match. He called back living beings from the afterlife without sacrifices or DNA. He created the Six Paths Chibaku Tensei with ease, a level of jutsu otherwise not seen. He was able to transmigrate by himself and appear as a spirit in the physical world.

Power over the soul is part of the Rinnegan, but Hagoromo took that to completely new level, hence his mastery thing.