It is true that the "surrender" option makes plundering a total free-roll. You can scout their city before choosing to attack, so at a minimum you can get a sense of what troops they're capable of owning, and if you happen to get a bad matchup, you simply resign and lose nothing. It certainly seems that for a plunder you should be forced to fight all the way through, or just surrender and lose all your troops. One alternative would be if you lose, they get to plunder you? Absent those changes, i think the most reasonable compromise should be that you can plunder whatever is available at the time. and once you click away from their city, you have to wait 24 hours.
I mean its so insanely easy to plunder, and you risk nothing, seems the least they could change is the ability to re-visit whenever and take whatever.