I ran into Russia coming at me with multiple armies of highly promoted helicopters, those things were nightmares to try and take down. The AI can be tweaked to produce more armies and be more threatening in a war. Not quite stacks of doom but better than a carpet of weak units.
I really like them for ranged siege, just roll one bombard army up and in one or two rounds the walls are down.
It took me till near the end of my third game to realize that I can produce armies fresh rather than building three separate troops and at a huge cost savings.