I run an Apex but use my heater and chiller’s internal thermostats also, to provide redundancy.
Main heater controlled by Apex with internal thermostat set just below the Apex ON temp. If Apex fails to turn it on the internal thermostat will. If the heater fails ON the Apex will turn it off. It the heater fails OFF the backup heater kicks in.
I have a question about this? The Apex uses the temp probe to turn power on and off to the outlet. If the Apex fails to turn on, the outlet will not have power and the heater will not be able to turn on. Your backup heater will still work though.
I run two 50 watt Jager's, both controlled by an Apex. I run two heaters in case of an individual heater or outlet failure. If the Apex fails to turn on, I currently have no backup other than an alert, if the Apex is not totally unfunctional. If the Apex fails to turn off, the internal thermostat of the heaters will turn off.