1) Such a society may have difficulty organizing in the face of an external threat.
Iraq faced a significant external threat in the form of a vastly militarily superior enemy with virtually unlimited resources.
Was government-organized defense more effective or was spontaneously-organized resistance more effective at preventing this external enemy from achieving its goals?
Great example. What's interesting about this example is that part of the reason the resistance was so effective was that it didn't have a command and control structure that could be seized. If you had to capture an AnCap society house by house, that would be a daunting task compared to just seizing the government of an already-conquered people.
Plus, it might not matter too much to people who rules them. If people don't see much difference between the two governments, then does it really matter that you were conquered. On the other hand, any conquering force would be a profound loss of freedom to an AnCap society. So there's more motive to defend. (For example, would Greek people care if France took them over? Maybe out of pride, but as a practical matter, there wouldn't be much difference.)
I don't know what "national defense" would look like in an AnCap society. But I have no reason to think it would be particularly difficult to come up with a way to do it.