NFS works great over the open Internet, as long as you do it through a secure tunnel. I've been doing this for years as a way of increasing the size of the available storage in a VPS.
It's really no different to running any other service over VPN or SSH tunnel. It does work well with NFSv3 but never tried with NFSv4.
I've used both vtun and WireGuard for remote NFS. On a good day, I get 100MB/s to the NFS filesystem on my San Jose AWS instance (from Vegas) via WireGuard. Note: That was before CoVid-19. CenturyLink/Quest has since (stealthily) throttled my bandwidth down from 1Gbps to ~750Mbps.