interesting. I did find them quite useless in defence battles for the reasons you mentioned: the enemy attacks you too soon to let your onagers really shine....and in my experience they hit cavalry only as long as the latter don't move.
maybe unit size matters? I'm playing on normal and they ARE useless while defending. Even if they manage to fire one of the two shots they have somewhat accurately, a very little percentage of enemy troops will be dead, and your onagers will be standing on the battlefield futilely until the battle ends.
about killing generals however you're definitely right. I have the same experience: Whenever you fire at units close to the general (and he will likely be close to some of his troops), he almost certainly gets killed by a shot. Of course provided you're attacking and thus have enough time to scrunch him.