I'm sure these were pretty dangerous, especially my big design, but I have seen too many images to think it was a failed prototype. If that were the case I doubt much would have survived. Its not much different than a big bow with one arm.
I do not think the major recoil, whiplash effect will act much on the projectile, since it will be on its way flying moments before the plank hits the rest. Its really like a well calibrated golf swing.
My design I made much simpler without an elevation aimer (i missed that part thinking it was like the bottom cart version), or rotating base.
not sure what they are using for a spring in these mobile ones. Maybe a giant coil spring.
This type uses torsion springs but same principle
Siege of Syracuse, approx 400B.C. look behind the first engine.
this is one I may make sometime.... 9 arrows!