Another thing that hasn't been touched on here is the ability of a band of "marauders" being able to move about freely.
The most powerful military in the world has real issues driving around Afghanistan without running into IED's that have accounted for over 75% of all casualties in the country. I would bet that with the resources and ingenuity of the USA populace that things will get real ugly quick for the bad guys.
Wouldn't take much to disable and kill everyone in a unarmored bunch of trucks.
If a bunch of Taliban can cause all the problems they do now against the most sophisticated and armored military in the world think what will happen when a bunch of marauders come looking for trouble in a Toyota pickup. I wonder how far in the air you could blast a pickup in the air with 50 lbs of ammonia nitrate and diesel fuel. Think Oklahoma City.
Bands of roving thugs are going to be cumbersome to move about, feeding and keeping them in armed is going to be a logistical impossibility. They will stick out like a whore in church and be less welcome.
My point is that your attacking thugs are going to catch hell long before they even get close.
That being said, I can't imagine the carnage in major urban area's.