The big issue is that 'terrorism' is not about inflicting harm, but fear of harm. A few quad rotor UAVs outfitted as you describe, possibly even with frangible

The big issue is that 'terrorism' is not about inflicting harm, but fear of harm. A few quad rotor UAVs outfitted as you describe, possibly even with frangible rounds or just dropping tiny flechettes such as those used from the air in WWI, would easily cause panic. For that matter, "suicide" UAVs that are either capable of delivering a lightweight shrapnel load without concern for the vehicle's survivability, or just using the kinetic energy of the moving vehicle and the rotating blades to crash into crowds, especially in some sort of flock, would discourage a lot of people from gathering in public places, going to clubs, sporting events, malls, tourist attractions, etc.