User blog comment:B214/Question and Answers 6/@comment-24098614-20170321225939/@comment-29963364-20170321230915

Probably somewhere between Ultimate-Class Devil and Satan-Class Devil.

I'm not really sure. We've never gotten any clear indication of what they're capable of.

Anyway, while the Sacred Beasts themselves may be very powerful, I think it's up to the possessors power and skill to make good use of it. Similar to a Sacred Gear with a Creature sealed within it.