The current ballista is much better than it was. I think there is only two dangerous enemies (Fulings and Seekers) and one exceptionally annoying enemy (Drake). If ballista could be used against just "enemies" it could be used against bosses too and that is not how it is meant to be used. The second problem is the number of missiles it needs if it tries to shoot everything.

Now you have to decide what enemies it tries to kill or rather alarm the player when those things get close to the base. Ballista's aim is so terrible that it cannot hit even slowly walking greydwarf.