I ask this question from a position of ignorance, but if you use step/dir with a modern AC servo plus appropriate driver, does that mean that there is a PID tuning issue but it is hidden from the user and is pre-programmed into the driver somewhere? Maybe some kind of adaptive self-tuning? Given that a servo system relies on feedback for positioning speed and accuracy, basic feedback theory says that the PID parameters will be relevant somewhere - just that they're not available to tweak?

I keep thinking about a CNC conversion for my vertical mill and although it could probably just use big steppers (except possibly for the knee) it looks as if AC servos are becoming available at more attractive prices these days.