Any equipment can fail at any time.
I have 6 RKL. All working for 2-3 years. No one failed ever. I also have an Apex because I can control things remotely.
A heater can get stuck and continue working. The RKL or any of the ones suggested from Amazon in previous postings can fail too.
But, if you have a good heater set up to, lets say 78-79 F plus you have the RKL set up to the same temp (79 F), then you have there a double temp control. Do I explain myself ? The probability that both fail are really reduced.
But !!!! ..... if you use the RKL , or any other controller as the ONLY control of your heater (like those heater bars, that work continually) then you increase your risk. You took out the benefit of a controller like the RKL. Redundancy in this case is increased security.
But again, there is always a small chance that the heater control and the RKL (or any other) fail at the same time........ that's life.
Best
Ps : in my SPS tank I have an Apex and a RKL. Lights, pumps, doser and heater in the DT are connected to it. The fuge is in another room. Has lights, Fans and more heaters. Those are connected to a RKL.
When an Acro frag or mini-colony can cost you something between $ 25- 100 or a fish $30, 50 or 100 + . Then you see an Apex not so expensive and a RKL as buying another good Acro.