The way I see it, it's not a matter of the heater being stuck on. It's that the 150 and 200w heaters get hotter in general than the 100w and lower models. So the 150/200w's will still shut off at the right temp, but let's say they get up to (just throwing numbers out) 150 degrees, which is enough to melt the epoxy used. Whereas the other models will get up to 100 degrees or less which is safe for the epoxy. A ranco will only shut off the power if the water temp gets over what it's set to be... and probably won't help here because the thermostat was fine on my heater and didn't warm past what it was set for, it was just leaking and dumping melted gunk into the water.
I hope that makes sense.