I built a NFS3-over-OpenVPN network for a startup about a decade ago; it worked “okay” for transiting an untrusted internal cloud provider network and even over the internet to other datacenters, but ran into mount issues when the outer tunnels dropped a connection during a write. They ran out of money before it had to scale past a few dozen nodes.

Nowadays I would recommend using NFS4+TLS or Gluster+TLS if you need filesystem semantics. Better still would be a proper S3-style or custom REST API that can handle the particulars of whatever strange problem lead to this architecture.