In order to secure NFS configuration files like /etc/hosts.{deny,allow} would have to be filled with correct configuration data. The current NFS NixOS module does not allow one to configure these, so how is one supposed to deploy NFS on NixOS?

One could argue that it is not responsible to even give the suggestion that NFS is supported in any fashion on NixOS today.

See https://help.ubuntu.com/community/SettingUpNFSHowTo for documentation for Ubuntu NFS setup.