iSCSI allows you to share physical storage but only *one* client (initiator) can access a given storage volume (LUN) while NFS allows more than one client to access the same file so even if you use an iSCSI device for NFS storage you still have the same security issues with NFS. Use of a proper capability design would allow secure access to network files but using it is nowhere near as easy as Unix's simplistic design of permission mode bits and shared password/group mapping.