I have exactly the same problem with an 18 FET controller I purchased from Lyen a month ago. Really bad grunting at startup, under load. Reverse pretty much useless. Baby the throttle and it starts somewhat normally, but start on any grade is impossible.
This controller runs very hot as well. With a 10a draw it can get too hot to touch. I've emailed Lyen but have received no explanation, only an apology that I didn't purchase an RS232 cable
Lyen sent a test video:
the motor he tested was not under load.
I tried a 48v crystalyte sensorless controller on this motor and startup torque was closer to what one would expect for sensorless. Slight hesitation until the controller found the zero crossing then normal acceleration. Lyen's controller handles this poorly, almost to the point of hardware failure.
Hoping someone out there can provide some insight into this