I have found Onagers to be highly effective in attacks. They are also effective in defense, but not nearly as effective as attacks. Usually, the enemy rushes in when I am defending, which doesn't leave as much time to hit them with the onagers.

But I will say without a doubt, they are one of the most useful units I use.

Also, it is not a 5% chance that you will kill the enemy general. If you have 2 or 3 Onagers and you target the enemy general unit, you have a VERY good chance of killing him. Hell, you have a very good chance of killing off his entire unit! I have not encountered many problems with the Onagers taking up 2 or 3 unit cards. Sure, I have lost once or twice while using onagers, but generally it is against armies where each unit has much more experience than mine. And I believe they were battles where I was defending, which makes the Onagers less useful (though by no means a complete liability).

I used to use flaming pots when attacking troops, but I have switched back to solid shot. I did this because the shot hops once or twice after it hits and the pots just hit one spot.

I guess I could set up some custom battles and try over and over and over just to see which is more effective against troops, but I am too busy with my Brutii campaign (hard/hard).