I don't know, I think this sounds disturbingly doable in the near future. Both your problems are essentially "opportunities to strike are few." But the point of drones is that they're cheaper than humans. If you can afford to tail someone with 2 goons, maybe you can afford 20 drones all along their typical route. And the drones can recognize license plates and other non-facial things to help them hone in.

As for facial recognition software, there's probably already an Android app for that. Sure, there's a lot of work involved - it has to recognize the person, it has to drive around autonomously, it has to aim and fire. But most of those may be pluggable components of hardware and software soon, coming from perfectly innocent projects.

I think drones may well be a big problem in the near future.