These devices are really good at monitoring things. In the long run, no matter what you do, you can bet failures will happen. I don't as a rule rely completely on one system, I have several units where If failures happen we have something else that will hopefully prevent totally destruction. The old timers would always say get two heaters with half the power needed & run them together, this way one or the other fails your still in check & shouldn't destroy your tank. The only problem with that is getting them both to run when needed & not more one over the other, the Ranco controller is commercially built, HD relays, which is why they cost so darn much but failure is seldom with these, the ACII doesn't have that type quality mechanical parts in the unit. Why we have return pumps separate from CL,s or PH's or whatever same goes with lighting & everything else, even though you can go days without lights, so I'd not consider that critical. Chiller & pump or supply water to that chiller is something that is overlooked. If you loose water to it & it comes on, guess what? That is why they sell these $150 flow switches that most people don't get. The software & program is the easy part & will run smooth when done right, but your perpetuals like, the relays, probes etc as Don mentioned aren't the same. Yes they work well, year or two may be fine but If you come up with a loaded reef with expensive fish & corals, more so as they grow & gain value, the extra 200$ you could of spent, just cost you a potential few thousands. Ok, on small systems, I wouldn't go crazy like that, but your nice average size or larger systems yea.
At some point you can get carried away on all this also, so simple can be beneficial, cover your critical systems & let the others fail If there are problems, as long as the potential to crash your tank isn't there, don't worry about it.
