The AI has a bad habit of placing units in isolated locations - hexes that it still controls that are surrounded by player-controlled hexes. Bad idea, obviously.
The AI also sometimes doesn't maintain a solid line of units across its front. In a recent game, some hexes in northern Belgium were left uncovered, and as a result the British were able to sweep around the German right wing and make their way all the way into Germany. Needless to say, this was crippling to the Central Powers.
It would be good if both AI and player could redeploy units to any friendly hex, not just those controlled by its own nationality. There are some work-arounds but it would be better if you could just decide that some hex is now going to be in the British or American sector or what have you. This is especially galling when you are operating an expeditionary army -- the Germans in Austria-Hungary, the French in Serbia or Italy, etc.
"There is no substitute for victory"
Depends on how you define victory.