I've seen onagers used to destroy siege towers. I think.

But honestly, I can't stand onagers. They're too inaccurate. Half of the time they go way off and even though when they do hit, the result is spectacular, it aint worth risking yer own troops for a little boulder lobbing. Scorpions and ballistae are alright but only of any real use when used in a group.

But, meh, since I'm Germania now, I don't have any time for artillery. It's spearmen all the way!