I recommend a herpstat or a VE 200 or 300 as a primary and run a johnson or VE100 as a fail safe. Proportional units can fail on or off typically so can an on of off. The failsafe is a proportional T-stat plugged into a on off the probes placed together and the on off temp set to a high end temp (like 95-6) and the Proportional set to 90 or so if it were to fail the on off will shut it down at 95-6 degrees.

No unit can protect its self against catastrophic failure.