These countermeasures generally only work against drones that are not fully autonomous since they rely on disrupting communication with a ground controller. If your drone is perfectly happy to loiter and kill people based on facial recognition and some basic parameters, dronegun won't work. The other point to keep in mind is that cheap lethal drones are often designed to operate in swarm mode rather than individually. It's pretty difficult to knock out an autonomous swarm with weapons designed to target one drone at a time.