One important thing to pay attention to is that the AI don’t seem to consider the player special, so unless you put them on the same team, they could attack each other and end up with nothing left to face you. It happened to me a few times:
The AI were so busy killing each other that they had nothing facing my own units and I could steamroll them easily.
It mostly depend on where you start relative to the AI. Putting the AI on the same team makes things much harder but much more consistent.