My initial thought when I first saw it was that it'd be interesting for its attack to only be able to target Conjurations. between itself and Arkiro's Hammer it would create the theme that the main function of War Machines is Anti-Conjuration devices and create a niche for it in War School (breaking down walls, blowing up temples, spawn points, totems, altars, death locks, wizard towers, outposts, etc). Arkiro's Hammer would then be the longer range, more expensive version which also includes a creature hitting zone attack while ballista would be a cheaper version that was only exclusively conjuration control.
Another thing worth pointing out about Ballista is that Reverse Attack stands a good chance of making a Ballista 1-shot itself on a decent roll due to its own heavy pierce rating.