There's a significant difference between using
ssh -X or the VNC system. With VNC you get a visual copy of the desktop on the remote system at that moment the VNC server was started. With
ss -X you start a new X-session; it also requires a X-Windows system on the local computer. Because a Mac is basically a *nix system, this shouldn't be a problem.
So it depends how you want to setup your remote desktop session: see what's on the remote monitor or start a new session.
ssh -X you should check whether X-Windows forwarding is enabled on the remote computer. Check the file /etc/ssh/ssh_config and your possible default settings for the entry
ForwardX11 (it should be
yes for this to work).
Then an important thing: once you are connected to the remote machine, all you have is a shell. If you want to see the desktop, start the Unity desktop (or KDE, or XFCE4) using the proper command. For instance for XFCE4 start
Remember that running Unity implies a lot of heavy graphics work that needs to go over the SSH-line, so perhaps you can install XFCE4 along with Unity.
Another tip: exchange SSH key for login-less SSH :) Use
ssh-copy-id user@machine and follow instructions.