On the right bike, coasters are fine if you keep the limitations in mind. If you break the chain a little toe jam braking is all you need, or you can always reach down and unplug the spark plug wire if you have good shock tolerance. I wouldn't want only coasters though, they make horrible drag brakes and don't (typically) work the front wheel where most of your braking ability is.
In my experience the biggest problem with coasters on a mab is that the heat dissipation is poor so if you're planning on stopping from 30 mph regularly you better have some grease ready
Personally, if I had my brakes and kill switch fail and the throttle stick wide open I'd just find some nice soft grass to lay it down in.