There are some playtesters acive on Octgn and most of them agree with giving Ballista the unique trait and allowing promo cards. After seeing at least 500 games on Octgn I can say that there is no Promocard threatening the balance in any way right now. Btw. As you can see in the current wizard discission, there are official released cards that have balance problems too ( wizard, wizards tower).
So check out the promos, learn what they can do. More cards, more fun, right?