A quality disc brake(like a BB5 or better) is the best, but good V or caliper brakes work well for most applications. The old time(1970s) short lever side or center pull brakes don't have strong enough stopping power for me. A rear only brake, of any sort, is inadequate for a motored bike, in my opinion. Here's the thing, I can pedal a standard bicycle at 10 to 14 MPH for a few miles and sprint at maybe 18-20MPH for a short spurt. I can ride my motored bike at 22 to 28 MPH. It takes much more force to slow efficiently from 25 than from 12.5, half the speed. The other problem is we generally have to ride in traffic with inattentive cagers at least part of the time. They don't always see us, and even when they see us, becase of our higher than "normal" bike speeds, they don't estimate our speed correctly. As a result we may need to use our brakes to stop very quickly in panic situations from speeds our pedal bikes were never designed for. Sure, a serious road biker or racer can sustain speeds in excess of 25 MPH for quite a long distance, but they are usually riding high quality road bikes designed for high speeds with brakes to match. We, on the other hand, are usually converting cheap, marginal quality, beach cruiser/comfort bikes that are(poorly) designed for 12 MPH sidewalk use and made to the lowest quality, cheapest standards. You are literally betting your life on the cheapest brakes the Chinese manufacturer can possibly put on your bike every time you ride your bike if you don't upgrade them when you put a motor on a bike and double or triple the speeds it was designed for.
In my opinion, a coaster brake is asking for an accident. They lock up the rear tire very easily when aplied hard in a panic situation with loss of control the result and if the chain slips off or breaks, you have no brakes at all. When you aply the brake, the weight shifts to the front wheel, making a locked up rear tire even more likely. If you buy a bike with a rear only brake, you are foolish not to add some sort a front brake, at the minimum.
I have a friend who rode his pedal bike across the country. On a long mountain downhill run, he actually melted the front tire(blowout) on the rim from caliper brakes overheating the rim. He lost control and crashed badly at high speed. Now he will not ride a bike without disc brakes.
I really like a disc brake on the front wheel, but quality caliper brakes on both wheels work fine for the terrain and at the speeds I usually ride.
I can't imagine anyone foolish enough to drive a car at highway speeds with brakes only on the rear wheels, or brakes that are substandard. Why would you risk your life with a motored bike with substandard brakes?
I agree with you on some points, but like i said earlier, it all depends on how and where you ride.
me for example...I never ride in traffic, hardly ever ride where there are a lot of pedestrians (I do see the occasional person walking their dog).
I personally do not use beach cruisers / mountain bikes to build motorized bikes out of (however, the majority of people here seem to use those platforms for motorizing).
A coaster brake is fine (for me) IF the rider knows how to set it up, adjust it right, and USE it correctly. yes, a coaster brake will lock up the rear wheel, but if you know how to control your bike in a skid, this can work to your advantage. just like defensive driving in a car, a locked up and skidding rear wheel can help you manuver out of the way of danger.
Now, i understand that not everyone is me, and a lot of people may not have the experience that i do.
I grew up racing bmx, and motorcycles, so it's kind of natural for me to know how to slide a bike around while maintaning control of it.
NOW, please do not take this like i'm saying that a coaster brake is fine, and it's all the majority fo the people here need...that's not the case.
where you ride and how you ride has a lot to do with it.
BOTH of my motorized bikes have caliper brakes (rear only) and one has a coaster brake as well.
BUT, I personally have never been in a situation where i was going to run into a car or a person.
I just ride for fun and I would never commute on a motorized bike (where the best braking set ups would be needed).
just my personal opinion, and this opinion may not apply to everyone.