How effective was Roman artillery in Field battle?

[Flavivs Aetivs]

The Romerschiffe finds had mounts for a Scorpion on the larger patrol vessel, so they were probably common on river patrol boats and liburnae. We know Ammianus mentions they could fire across the danube, which was proven with the "firefly" device over in (Hungary?) So they would have been useful in things like river crossings. Also made for a good scare tactic.

Later on there were supposedly crossbow-versions of them used in the late roman army, but there is limited evidence of that.