X11 forwarding is as secure as your SSH connection as everything is handled over that as long as you trust the system you connect to as it can send some X11 commands to the client. VNC by itself is insecure but XPipe tunnels all VNC connections via SSH as well, so it is secured as well. With RDP, I would argue that there are less sophisticated authentication options available for RDP than for SSH.
I think moonlight and sunshine are intended for gaming while this more intended for server administration tasks.