All three units control everything in the tank they control, except the lights. I have one of the RKL's controlling the ato, tunze on the other 2 ato's. My tanks don't seem limited by the Reefkeepers, I keep whatever fish and coral I want!
I have no experience with an Apex system, and unless/until my Reefkeepers do something terribly wrong, I won't.
Digital Aquatics is about to Launch the Archon which will be light years ahead of the RKE. I have and RKE and love it. it does everything I ask it to reliably.