An Edgedancer sliding past enemies, taking swipes at them with their spren blade as they pass. A Lightweaver gets sliced to pieces only to actually be in another place, the dead one nothing more than bent light. Elsecallers turning enemies into lightning or flame as they teleport across the battlefield. Truthwatchers healing their comrades and popping up illusions to distract. A Willshaper teleporting around making enemies and their weapons as loose as a noodle, unable to fight.
I just don't see how how they won't make effective fighters.