There is a "cross-over" point in power requirement where servos start making more sense that steppers - something around 400W, IIRC. Below that point, for "typical" applications, like a milling machine, a stepper will do everything a servo will do, performance-wise, and at a lower cost. Above that point, steppers can no longer deliver the performance, and servos start to make more sense, albeit at a hggher cost. But, 400W is FAR more than is required for a BF20.
There are always applications at both ends of the spectrum for which that rule does not apply, because of unusual requirements, like extreme speed, or something similar. The trick is to design the drive system to fit the application, not just blindly mix and match components. It takes actual engineering to do it right. But, for milling machines and routers, the rule works pretty well. Yet, most people still seem to believe that lost steps are an inherent characteristic of stepper motors, and servos will ALWAYS perform better, and never lose steps. Just ain't so....
Regards,
Ray L.