User blog comment:FairyRaven/Fairy Tail Chapter 357: The Nine Demon Gates, Review and Commentary/@comment-112.208.2.201-20131025102910/@comment-7418318-20131025140317

My guess would be that each single one of the gates was strong enough to finish any of the missions and thus we can't judge their strength by which mission they were assigned.

We can't even judge that Demon's strength based on their attacks, since almost everyone villain in Fairy Tail started off with weak attacks and only started to reveal their true power bit by bit when the minimal-force approach didn't work.