Been trying out onagers in bridge battles recently, and I still think they're something of a double-edge sword. Flaming missiles are pretty good in chipping at the morale of the enemy, but honestly, other than taking down walls, there's nothing they can do that a couple of archer units couldn't do, better. Worse, in one of my battles, I had two units of Spartans guarding a bridge, I lost half my men in each unit. Not to the enemy, but to flaming missiles that misfired. I was extremely annoyed, but luckily they were new units, so I didn't lose any veterans.

One thing onagers are really useful for though: destroying enemy artillery, and elephants. Nothing more gratifying than to see the feared troop of war elephants run amok in the middle of an enemy army....