The concern is not whether a backup system can be built, that technology has been out for decades. Rather, it is whether you can assure yourself that a backup system will function and function correctly after any of the failure for which it is required. That is much more difficult to prove than whether the backup system, once activated, will do its job.
I wouldn't bother comparing UAVs to GA, two very broad fields. Perhaps comparing hobby R/C to GA is closer, but now unfair to GA. The technology we're discussing is the replacement of professional, legislated, and experienced personnel/equipment.
This isn't an overly safe, pessimistic view of certification. UAVs crash, and most have a much worse safety record than manned aircraft. A sample from different countries:
MQ-8 Fire Scout UAV resists its human oppressors, joyrides over Washington DC -- Engadget
Glitch Reporter: Predator UAV Failure
CU161009 Sperwer... | Uninhabited Aerial Vehicles | Reports - Investigation | DFS | Air Force | DND/CF
1 of Two RAF Reaper UAVs Crashes in Afghanistan - Defense News
T-Hawk UAV Fails at Fukushima | Japan Probe
ADM: Shadow tops UAV crash list