We probably need a manpage to define what sysctl nodes the usb ethernet driver has, what usb devices it support and the kernconf options/device required to build.
Doesn't ue.4 just need to redirect to cdce.4? The only sysctl in the net.ue OID namespace is the parent device, which provides all of the actual functionality. cdce(4) looks to me to be what people might be seeking for this device; https://man.freebsd.org/cdce(4) definitely has all of the information you're asking for, as far as I can spot.
*** Bug 252856 has been marked as a duplicate of this bug. ***
ueX is common to all USB ethernet devices, from what I know. --HPS