In short, the thermostats in most heaters cycle a lot and are relatively cheap ($30ish gets you everything, heater and controller). I run an Inkbird thermostat to switch on or off my solid state titanium heater from BRS. Previously I ran Eheims for 2 years, then replaced them out of fear of breakage. I had not had a bad experiecne myself, I just heard enough horror stores to make the switch. I would test your heaters and internal thermostats for sure before relying them for temperature control. My view was assess the cost of the heater, vs the cost of a heater failure.
Vinegar or citric acid to get the scale off.