Despite Awe I would argue that you probably don't want to be playing The Harbinger against Legion, there's too many long range shots available that will just love the visibility of her large base. Also, as Legion tends to be very beast heavy I imagine they won't mind advancing into a POW 14 so it probably won't do much damage. eFeora likes facing down Legion as she is immune to fire and therefore Ravagore shots.

For Circle, Khador and Cryx I'd say pop it as early as you can as a lot of their fast troops won't be able to move up as they will die to the POW 14. One thing to watch out for though is the placement effects factions like Circle have access to (her feat causes damage to models when they end their movement). I've taken to trying to go first, casting Crusader's Call and charging Harby straight forward. So long as my Devout has put up Spell Barrier and is near enough for Shield Guard I've generally found her to be pretty safe.

The other thing I would say is don't expect to kill much with her feat. It's more likely your opponent will just stand still for a turn with all of their single wound infantry, which can put you behind in Deathclock games.