What is the effective speed-control range of a BLDC motor?
When Dc motors have a no load speed proportional to average voltage, you can reduce the no load speed accurately, but since torque is dependent only on current, it will not be stable be varying loads. 

Also since the starting torque is limited by Vdc peak and DCR of coil and switch, it is usually 5~8x the max average current at rated load and speed. 

Thus the speed regulation drops since the apparent impedance of the motor rises at no loads due to back EMF.   


  When you are at any speeds it is  like a voltage regulator with a series R, its output V will be load dependent just as motor speed becomes load dependent. But a lower speeds , the load regulation gets worse and DC source impedance adds to regulation error. e.g. A PWM is open circuit when off unless it is a complementary driver. So an open drain or open collector with PWM raises the source effective impedance and makes load regulation worse.


The % Load regulation is determined by ratio of source to load resistance in % V and in the motor is the ratio of available torque(I) to load torque for %speed variation.

If it stalls then it becomes a short determined by the DCR of motor coils and is determined by the rated stall/starting current and voltage applied.

Thus a rotary encoder is needed for precise RPM control in a servo loop.