User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170506225314/@comment-29963364-20170507045102

Actually, the fact that Sirzechs needed multiple Pieces (and a Mutation Piece) to reincarnate several of his Servants says more about their strength & potential than Sirzech's.

It was also stated in the series that Sirzech's peerage is the "strongest group in the Underworld."