The other day I couldn't RDP into an iMac, I could issue remote commands, I could ssh in and do things, BUT no RDP through ARD or even VNC...turns out that if "Sharing" is disabled under restrictions that that breaks RDP. 
I guess it kind of makes sense but I always thought of restrictions in the sense of restricting user access to settings, not disabling features. 

Just in case anyone else runs into a similar thing...

Best,

Phil