If the ballista fires shortly after the hero's turn, rather than when the hero is almost ready to move, we don't lose much. But even if we do lose something, it doesn't seem that bad. If I'm wrong about anything, let me know.
The hero only gets to move once – when they load the arrow (and the ballista accordingly) – after which either the ballista fires (and the hero doesn't move), or the hero moves (casts a spell), but the ballista doesn't fire (e.g., by using the "Wait" command).