I've said this before about the accuracy of these devices. I can put a few tracers into the mag and pull ~20 of 30 rounds into a car door at 200 yards. If you don't think this is an effective deterrent to a mob of brain eating zombies coming for you in a SHTF scenario I can't educate you further.
A little tidbit from a Kosovo story I read. Armed thugs held the bottom of an exit ramp and were roughing up & stealing everything from everyone tryng to pass. The person in the story had valuables he didn't want to relinquish & young kids to protect. He stopped at the top of the exit ramp. Thugs lobbed rounds at him (200-300 M) so he returned fire with a fully automatic weapon nailing more than a couple. The thugs scattered like cockroaches. He was able to get himself and family past their roadblock and on to safety. If you think something like this won't go down in a SHTF scenario here I can't educate you further. A bumpstock might be the only device you have to perform this type of task.
How long would it take you to put the same number of rounds through the same car door with aimed semi-automatic fire - and would you still need the tracers