I am highly opposed to the use of this "impact ballista" for a few reasons:
1. The Nomads live in a desert, where wood is rather scarce. They probably use the wood for their structures and tools more than stuff that requires a lot of wood such as a ballista.
2. The Romans already have a ballista, so we don't want to be redundant.
3. IRL, ballistae are very inefficient at taking down buildings. They are excellent anti-infantry weapons, but are pathetic when used to take down things made of stone or strong wood. Now, before you say "but these have rocks on the tips!", the weight of a rock would severely weigh down the projectile, causing it to become very impractical. As you can probably bet, the Romans (who were one of the few large armies that used ballistae IRL) eschewed ballistae for anti-structure use and decided to use something that was actually effective, such as a catapult.
4. IRL, the Chinese were the first to actually start using cannons and gunpowder, but that technology was soon acquired by the Muslims (who then made it much more practical). It would make total sense for a desert faction to use very primitive firearms as weapons. Anyone with any concept of history (something we're sorta basing this refit on) would be expecting the desert guys to be using gunpowder.
I had suggested that we go with a Hand Cannoneer instead for a few reasons:
1. They are historical! Before the advent of easily portable firearms (such as the arquebus or musket), hand cannons were used. They were small enough to be portable by an infantryman, but powerful enough to do damage.
2. They make for interesting gameplay! Rarely do I find a faction's siege unit to be an infantry. I think that this would make for a very interesting unit. He'd probably do less damage than other dedicated siege units, but would be more mobile.
3. It fits the Nomads much better than a ballista. I assume that if we did have a cannon unit (whether that be a bombard or hand cannoneer), it would probably be purchased from a market (seeing as the Nomads have ties to people with the ability to forge such great weapons). So yeah, the cannoneers would be mercenaries.
Honestly, I don't care what they use as a siege weapon, as long as it's not melee (such as the Njord's) and it's not that stupid, out-of-place ballista.