My last comment to this post seems to have disappeared in the switch, along with some other comments on other posts. Every one has opinions and to demean an other's choice is just hardheaded. Let's try to keep our minds open and to be civil, please.
My post is a warning about a certain brand and model of cheap, very low quality disc brake calipers that come as standard equipment on low cost Chinese made bicycles. The problem is not with disc brakes in general, but only with cheap, low quality ones. A high quality rim brake that works is far better than a low quality disc brake that doesn't work.......... period. I have no experience with bicycle drum brakes, so I will not comment on them. I will urge people with just a coaster brake on their bikes to add a front brake of some sort. The front brake does the majority of the work and in a panic stop, a rear only braking system is dangerously inadequate. A single rear only brake or poor quality brakes in general could easily land you in a hospital bed, a wheelchair, or a coffin. For years I have been saying buy the best quality bearings, rear wheels, tires, and brakes you can afford for your motored bikes. I stand by that statement and now I'm heeding my advice.
Grabbing two handfuls of brakes hard without effect is distressing, to say the least. My bike now has a rim brake on the rear that works fine and a very good disc brake on the front that also works fine. Use whatever type of brake that works for you, just use high enough quality brakes to stop you quickly with control EVERY time.