I'm in the process of overhauling my mill using the same components. I'm using a PMDX-126, PMDX-107, PMDX-108 (input), and a UC400ETH. Currently running Mach3 with the UC plug-in, but I'll be getting the UCCNC license shortly.
I'm not done with everything, but I now have the axes moving under PC control, and the spindle is turning.
The PMDX components are top notch. No hardware problems at all, and the UC400ETH is communicating properly with the PMDX stuff - including the 108 input board. I am having problems with the UC Mach3 plug-in, but the UCCNC folks are working on it and will hopefully get it sorted after the holidays.
Problem 1 is that M03 and M04 commands do not change the DIR pin state. Both commands rotate the spindle the same direction. It's not the UC400ETH, PMDX gear, or the VFD; it's a software problem. UCCNC support gave me a workaround (that works!) until they can troubleshoot the plugin.
Problem 2 is that the spindle pulley ratio feature is not working, and I suspect this is also the UC plugin acting up. Doesn't matter what ratio I set the pulley to in Mach3, the "S" command results in the motor rotating at the command speed (not the spindle).
Using a higher-end Monarch strobe, I was able to tune the PMDX-107 to within 0.5 RPM of the target speed on my motor/VFD combo under 'Test' mode. Dropping the RPM in 'Test' mode to 30% mode was within 1.0 RPM. There was essentially zero RPM fluctuation, and I'm pretty satisfied with that. It should get even better when I get around to installing my spindle encoder.
You might consider going with the UC300ETH and motherboard instead of the 400. CNC4PC is still offering a free UCCNC license with the 300, which means it's actually cheaper than a 400 + license. I was a little annoyed as this deal was offered about 2 weeks after I bought the 400, but such is life.