It sounds more like a driver error/experience problem. Also, most people who rent a U-haul trailer don't have trailer brakes! I can see where you wouldn't want to install them for a 1-time tow, but U-haul should first see whether or not the drivers can handle a trailer or not. They must get a surprise riding their car brakes down that first hill and find that they don't work when they reach the bottom, and so therefore flip the car. Also, most people don't pay much/any attention to overloading (BIG MISTAKE!!!) and simply overload it.