The downside is that control circuit is much more complicated. Where DC motors have a commutator that switches the coil voltages, a brushless dc motor has no commutator and the voltages need to be switch externally. Which means you need a way of measuring the rotor position. Two ways to do this, with a hall effect sensor, or sensorless by using the back emf of one of the coil windings.
An App note that explains how to do this using a PIC
http://ww1.microchip.com/downloads/en/AppNotes/00857a.pdf
The motor that used was a Pittman N2311A001
http://www.pennmotion.com/pdf/est_bulletin.pdf