I went down Mt. Lemmon using only coaster brakes. Reached speeds up to 40mph+ and had no problems. I did add a front v brake and also v rear brake but only for safety. I only used them a couple times when I got going too fast and did not want to heat up the coaster brake.
Mount Lemmon Ride pictures by FR31MB - Photobucket
YouTube - Motorized Bicycle up Mount Lemmon in Tucson,AZ
My rear wheel is a BICYCLE WHEEL 26X2.125 WITH COASTER BRAKE 12G SPOKES: Husky Bicycles