Just about all heaters out there will not be accurate with respect to the dial or setting knob. Some are better than others, but none are 100% accurate. It doesn't mean the heater is bad or thermostat is failing. It just means it's not accurate.
With that said though... there is a recall going on with the Marineland Stealth Pro heaters and you should NOT have them in your tank. The Stealth Pros are the ones that are totally black, with the red/green led
-lit knob on the top. These are the ones being recalled. Marineland will replace them, and if you ask for a Visitherm (glass) heater, they should oblige. The original Stealth heaters are black, but no indicator light on the setting knob. The knob to set the thermostat is just red plastic. Those, in my opinion, are good heaters.
Personally, I'd never trust a thermostat on ANY heater. If you want a more reliable setup, get yourself an industrial temperature controller (Ranco, Johnson Controls, or a full blown aquarium controller) and use IT to turn your heater on/off. Set the thermostat on the heater just a little higher than the controller set point, and you have a 2nd line of defense from overheating. That's about as bomb-proof as you can get.