I've never put together a CNC system, but I do know that not only the drive, but also the control scheme is very different for AC and DC servo systems, with AC being the more "complicated".

The DC motor uses very simple duty-cycle-controlled PWM to control the motor the speed of the motor, whereas the AC motor uses sinusoidal PWM, where the speed control is in the frequency of the PWM -- enter the variable frequency drive.

AC motors used for positioning applications often use the "Vector Control" scheme. I don't know much about it, but here's a paragraph from "Electric Motor Drives" by R. Krishnan.

"Separately excited dc drives are simpler in control because they independently control flux, which, when maintained constant, contributes to an independent control of torque. This is made possible with separate control of field and armature currents, which, in turn, control the field flux and the torque independently. Moreover, the dc motor control requires only the control of the field or armature current magnitudes, providing a simplicity not possible with ac machine control. By contrast, ac induction motor drives require a coordinated control of stator current magnitudes, frequencies, and their phases, making it a complex control."

I don't claim to fully understand the implications, but I understand that controlling the phase of a signal (AC drives) is far more involved than controlling the magnitude of a signal (DC drives)

Hope this helps.

Chad