If you're talking of UCNC devices running USB... I can guess at a couple of potential issues.

You're running ethernet to the control, from the control to the pi, from the pi to several USB UCCNC devices?

I believe (though I could be wrong) that UCCNC will enumerate the device that it's connected to to determine the type of interface. If this is a USB interface then there's a chance that "Computer says No" when it examines the device and wonders how it's talking over ethernet. There's also a chance (and I could be wrong) that devices that support USB and Ethernet interfaces that the message encapsulation (and protocol) will be rather different.

Really daft question, why don't you use ethernet-compatible UCCNC devices instead of USB?

Now, possibly you're talking USB MPG pendants and similar?, and not the UCCNC device? in which case I see fewer problems, but much depends on the individual devices and drivers. You're unlikely to saturate the ethernet though I would suggest running a private local LAN for the machine traffic (including USB) - I wouldn't share this with home network (so, twin-pic configuration) - but that just for common sense protection.

Ethernet isn't USB (I don't mean that to sound so trite) but the behaviour of peculiar interface implementations may not survive well over the format translation. I'll be interested to hear how you get on. I'd hazard a guess of 60-70% chance of success.

Personally, I wouldn't, but that doesn't mean that it can't be done.