User blog comment:Real Godisme/Chapter 481/@comment-4933386-20160411124536/@comment-4933386-20160412073101

Well Mard Geer not being able to be summoned might be because he's not really dead... just revert back to his book form.

Buut wow I think that a dragon is still superior to all those characters put together, including the demons. Given that a god wasn't even summoned, maybe it's just exclusive to summoning humanoids (demons and human wizards)