I've been waiting for someone to make man portable tiny radars and small diameter rockets for knocking quadcopters down. They're slow, low altitude, and fragile.
But how could you ever blanket a city? Radar has shadows. You can use terrain and structures to conceal your approach. I have yet to see the radar system that can pick out a DGI flying below canopy level through a forest or between the floors of a parking garage.
Good question. I could see bad guys pounding a town like Houston and putting refining out of business for a while in places. It's much worse than hitting the grid even though there is no immediate problem for most of the country. Electrical guys are pretty good at repairing damage that I've seen. Cleaning up, replacing damaged parts, testing and then restarting a refinery looks slow by comparison. That's not counting green crazies and their lawyers suing to keep you from rebuilding.