That's fair. It could just be that they aren't a powerful enough spellcaster to deal with anything that Maleficent puts effort into -- Ilphyl can undo things she doesn't intend to last early, but they can't do anything against a curse she intended to be indefinite. (Or can only temporarily block the effects as long as they concentrate on it.)

Or we curse Ilphyl so they can't access their own magic or undo her spells.

Or both; if Maleficent realizes that someone among the Corsairs can even touch her curses even if it is not enough to undo them, that person becomes a priority target.