I suspect the problem with Mach and external motion controllers is to do with how oem/userDROs are allocated and updated.

I know when I was still running Mach3 on the lathe, I couldn't use the threading wizard directly, as I unknowingly used the same UserDRO for transferring the current turret position from the KFlop to Mach, that the wizard used for the thread pitch (it was ok if the pitch I wanted happened to match the current turret position though!). It took me a while to figure that out, but by that time I was in the habit of generating any required code on the laptop anyway, so never bothered changing what userDRO was being used.

The hardest part I always found about writing Mach macros, was finding out what all the DROs were meant to contain, and I'm sure certain plugins used DROs with no documentation to tell you what DROs were used, let along what they were used for. All it would take was for you to use a DRO that was already being used, and you had the potential for things to wrong.

Thankfully I've never had any problem with probing using Mach3 and a KFlop.