Onagers really inacurate at the beginning indeed (talking about campaign) but do get better with experience, something like arty in MTW. I first used one unit during a siege, making them attack the wooden palisade and gates. Pretty soon, the whole city was in flames, buildings crumbling in clouds of dust, while the fence stayed shining brand new. I kept looking in awe the weird trajectories of flamed projectiles, being smart enough to maintain my units at a respectable distance back from those diabolesque machines... Errr... the advisor said I should not destroy the city, so I thought I'll finally get a nice and large courtyard surrounded by a lucky fence AND the meprise of the advisor.
Slowly, the onagers started to hit the gates and busted them.

I kept using the onagers the following battles just because I was always short of units, confronted with larger enemy armies. They did improve. They finally become my favorites when pulverizing in flames one greek enemy general that was leading an army outnumbering me 2:1. I love them now.