I never have issues with Mach3 and when I say never I mean that. I have had issues but that was hardware failing such as the SmoothStepper eventually dying (it was a beta Ethernet SS)
I did switch to an SZGH on the lathe and love that for a lathe but for Mill and plasma I probably wouldn't go that route. I still have Mach3 on the Chiron FZ12, Beaver NC5 and the plasma.
One of the most common problems I encountered with users having Mach doing weird things was because they were using a pirate licence. Now before people flame me I am not saying everyone that has problems is using one, I a simply saying it was the most common.