I don’t use or trust combined heater/thermostats that use conventional bi-metallic strips. I believe that some modern combined heater /thermostats use electronic thermostats today, and do not rely on bi-metallic strips anymore, but the majority of the cheaper modern combined heater/thermostats still use the troublesome, unreliable bi-metallic strip, which have the habit of “sticking-on” and over-heating the aquarium with disastrous consequences.
I use either Eheim Thermofilters (I’ve had of these fail, and over-heat an aquarium!), Hydor External heaters or dedicated external thermostats connected to a heater to control the temperature in the aquariums in the fish house.
I found a box of Armitage Combined Heater/Thermostats in the attic from the 1980-1990s, I don’t think Armitages, Nottingham manufactuer any electrical hardware for the Aquatic industry today. I’ve spent some time removing all the bi-metallic strips, and hardwiring the heaters so they are permanently on, so they can be controlled by my dedicated external thermostats. I was going to throw this box of heaters on the local tip, so they now have a new life.