A flock of such air-crafts in a fleet formation if used for massive offensive action (like the Luftwaffe-style bombings in WW2) could be "switched off" with an http://en.wikipedia.org/wiki/Electromagnetic_pulse in the most severe case. Besides this extremity - a distributed but single-coordination-and-control battery of modern ground mobile anti-aircraft defence rocket complexes can deal with tens to up to a few hundreds flying units (http://en.wikipedia.org/wiki/Pantsir-S1 ; http://en.wikipedia.org/wiki/9K22_Tunguska).
I don't think that in the observable future these drones could be something more than a single-plane single-mission reconnaissance tool used strictly against technologically vastly inferior armies/societies.