Yes, a critical failure of the flight controller or power system can still cause it to fall from the sky. I've never had that happen personally and I've logged a lot of hours on custom built quads.

If you are using alpha/beta FC firmware or your settings are still being tweaked you shouldn't anywhere near people anyway.

I've lost engines that resulted in nasty crashes. As I fly quads almost exclusively they are always nasty. As such, I never fly over or near people (other than myself).

I've read of full power loss, but never personally experienced it.

In any case I think I'd be more afraid of bad FC firmware that resulted in a fly-away event. Those are super dangerous as the machine just takes off into the air and away from you. That has the potential to be MUCH worse as the altitude they eventually fall from is significant. If you've never seen a full-power ascent of a multicopter then you're in for a surprise at the speed at which they can disappear while going straight up.

Once you have redundancy in the powertrain, getting redundancy from a flight computer is relatively straight forward. At that point 3 rotors on one side of the craft or multiple computers have to all fail simultaneously to cause a real loss of control, and even then algorithmic changes can be made to the flight computer to allow semi-controlled landings with multiple prop failures.

I would think a multi-FC voting system, like what the shuttle used, would be the way to go for FC redundancy.

Mix that with a dual power system and 6 or 8 motors and I think you get MUCH better redundancy and fault tolerance.

And while you are at it perhaps a disconnected drogue chute that deploys if a watchdog signal is lost. Not something that is large, just enough to slow the decent to non-dangerous speeds.

that would be a much safer setup but don't see these consumer drones implementing all that!

A consumer follow-me quad with a youtube camera launched via kickstarter? Probably not. A future DJI aerial photography product with a 4000-8000 price tag? Absolutely. If Amazon ever intends to deploy these for actual package delivery instead of just PR they'll need redundant computers with a voting system, fault tolerant flight control algorithms with some kind of fault case detection and some kind of failsafe parachute / alarm system to slow down descent and alert anyone on the ground of an incoming object with 16+ razor sharp carbon fiber blades.

To say nothing of things like active sonar/lidar avoidance and a secondary navigation system of some sort for when GPS is unavailable.

Absolutely, for the higher end. What scares me is these consumer drones with inexperienced pilots/autonomous tech without proper failsafes. I'm pretty sure they will be banned from ski slopes etc pretty quick.

Optical flow can hold pretty stable when GPS is unavailable, other types of CV like SLAM even better.

"What scares me is these consumer drones with inexperienced pilots/autonomous tech without proper failsafes." For people who are into building and flying multirotors in a hobbyist capacity-- this is the nightmare we live everyday since DJI showed up.

Optical flow can hold position pretty well-- but will they be able to figure out how to navigate by it!?

For autonomous navigation you need SLAM and some AI. We're not there yet but won't be too long. But yeah it won't be safe in crowded areas