Hello,
With the Geckos or Rutex servo drives, the encoder feedback loop is closed at the drive. The software (Mach2 etc. or any other Windows/DOS based cnc software) has no clue. The software says move some number of steps/encoder pulses, and simply has to trust that the motor moves that far. If the motor gets bogged down or even stalls out, you will have lost your machine home and part zero postions. Geckos will fault out with +/- 128 encoder counts, I *think* that you can rig the software to read the fault and go into e-stop, but that's it. Any errors within the +/- 128 counts is simply lost motion (accuracy). The Rutex drives have a much higher error trip point +/- 20,000 counts. If you calculate your resolution (inches per encoder count) you can see how much you would be off. For example if 1 encoder count is .001" then you would be off .128" (a bit over 1/8").
Linux EMC does have the capability to close the feedback loop in software with the right hardware. You can also run open loop (closed at the servo drive) like above. The most cost effective way I've seen is at
http://www.pico-systems.com/motion.html
These boards only work with EMC, but have some awesome features. You would be interested in the Universal Stepper Controller and the Gecko Servo Interface. You wire the encoders to the USC and then to the Geckos. EMC can read the encoders from the USC giving true closed loop. It also acts as a real DRO with no power to the motors (if you leave handwheels on your machine). It also has a hardware pulse generator giving 300,000 pulses/sec (vs. 45,000 p/s or less for Mach2 or similar software pulse generators with a fast computer which can greatly limit your max speed if your encoder resolution is high).
If you want to run in Windows, you are looking at $1000+ IO boards, VERY expensive software and expensive servos/drives (no step/dir Geckos/Rutex drives).
I don't know about liner scales. I've heard the DRO types you see have to slow of an update rate to be of any use for real time CNC.