I'm not sure how much of this is bugs and what is not, but here is what I found from my testing of catapults. If you want to explore and discover by yourselves, warning, spoilers ahead.
Normal Ammunition - All seems to work just fine.
Shrapnel Ammunition - I feel like this one is probably bugged. It's more expensive then the regular ammunition, but does significantly less damage. The description says it should damage things that move near the impact, but from my testing (moving into the impact area a few seconds after the shot landed), this is not true and the shot has no secondary effect that I was able to detect.
Incendiary Ammunition - I'm less sure that this one is a bug, but it has a strange behavior. The big awesome-looking storm of fire VFX is not actually associated with any damage, which feels weird. The fire damage only starts applying the round AFTER the VFX has played. So you have this big impressive visual that feels like it should hurt a lot but doesn't, and then you have damage without only a much less impressive visual effect.