When using a 10.12 machine as the viewer, connecting to a 10.11 machine as the host, it's completely unusable. Artifacts all over the screen when you move the mouse cursor. This is using the built in screen sharing on both machines. However, a workaround (not ideal) is to use the free VNC Viewer on the 10.12 machine. It works, but you lose the shared clipboard that the native Mac OS supports.