Yeah I tested it out. The feature in ARD that I am referring to is that you can do any of the main functions (observe the screen, copy a file, install a file, run a terminal command, shut down, etc) to any number of computers at the same time. You just shift click command click to select multiple computers and then press the button associated with that function. That you can't do is control multiple computers at the same time, since that wouldn't really be practical, but you can observe all of their screens and then double click on one of them to start controlling it.

Really the most useful thing here is the ability to send an arbitrary terminal command to multiple computers at the same time, which you can't do in Remmina.