anyone noticed that heater have internal thermostat that switches them off, while the

[Simons]

yeah I agree, one of the most common tank crashes results from a 'stuck' on heater.

I stay away from the internal switches, and use titanium heaters, when they are powered they are on and I have two temp probes to verify shut off temperatures.

Even if your Apex fails, the socket will fail with it's "SET" condition, so if they are SET OFF and fallback is OFF an Apex failure shouldn't ever allow them to be 'on' by error.