I suspect that the underlying UDP ethernet protocol is nearly as error prone as the USB protocol, however, the TCP/IP layer which sits above it and does the error correction is (as has been pointed out) extremely robust so the end result is much better.

I can see both sides of the argument but theory is worth nothing if it doesn't work in practise...

For what it's worth, I've got a 3D printer which is extremely noise sensitive on the USB connection (to the point I now download the print to a memory card and print from that) and a USB CNC controller which I haven't had a problem with but there are dire warnings on the website about using a good quality cable - see https://www.youtube.com/watch?v=NUu9xwDfJ9k