The AI is pretty bad on land, but after playing Japan/US a few times, I have found that its biggest failure is at sea. Big powers often split their navies into bits that are too small to do major damage to big fleets, but are big enough that they are huge hits to a nation's naval power when lost. Many times have I taken out the USN/IJN without even losing a capital ship of any kind.
Ive also found that the British AI has some odd fetish with Airplanes, and will devote a majority of its resources into having an airforce of around 30000+ planes by 1945, but still cant deal with paratroopers for some reason.
