Except that it needs fuse and it does not come shipped with RHEL. So in this case NFS over SSH is simply easier.

Besides, to be able to do sshfs from that isolated server, you need to tunnel again through your existing SSH connection. So that would be sshfs over SSH, which has somewhat more overhead than NFS over SSH.

But in general sshfs is a viable alternative for remote mounting and for that reason I have it packaged in RPMforge as fuse-sshfs.