I played as Erebor against a medium level AI Isengard in WOTR and noticed a couple major issues. Several times the AI did not build on any build plots at all, so that, obviously, as soon as I got rid of their troops they lost pretty quickly. Other times, though, certain areas would cause them to lose when I took out their troops even if they had a citadel and barracks built. The most unfortunate example of all, though, was when I attacked their capital, Isengard. The AI had two large armies with two captains, one captain and it's army as reinforcements, of course. I had three. I attacked, they sent out all of their first army, and as soon as I wiped them out I won.... I never even got past Isengard's walls, let alone destroyed Orthanc.