I'm using Mach3 with an Ethernet SmoothStepper on a commercially-built mill. I probably get about one Mach3 crash a week - error message always associated with ESS buffer problem or unexplained ESS problem. Hasn't yet broken a tool or written off a workpiece but I'm sure it will happen. And the Mach3 "run from here" is a nail-biting thing to use...

I'm now putting the ESS docs and UC300 docs side by side to look at the difficulty of doing a brain transplant. I have a feeling that with some custom cabling, it might not involve major reengineering.