Reliance on a single coaster brake is dangerous. When a coaster brake fails, it can fail in two ways that are both very dangerous: full lock-up or full brake failure. They also have the tendency to catch at strange points in the pedal movement, like the vertical noon/6 position, which are difficult to establish the needed force to slow the bike.

I have a Shimano Nexus 3 speed with a coaster (and hand brakes) that is my daily driver, and where the coaster brake catches is pretty random. I can sometimes make several, as in 3 - 5, full backwards pedal rotations before it catches, other times it catches immediately. I am now in the habit of catching the brake before I descend any large hill. Since I live in Nobb Hill SF, that's every day.