I haven't met an AI in any 4x tbs that doesn't select units with a large degree of randomness. MoM, dom3, homm, aow, you name it. There is no real good alternative, and the more special abilities there are the more difficult it is for the AI to assess. The only alternative I can think of is what they did in shogun2, where the AI simply instantly spawned pre-made armies inside the fog of war as you were exploring.
It doesnt, actually... they are hidden, and there is only a limited chance you will see them. The AI knows this and abuses this. I've marched up to an army, gotten ambushed, reloaded and walked a high-spot ninja past and located it the 2nd time around, two turns earlier. No spawning, just sneaking.