The SH-27B can overheat. The SF-28 will not.

While the SH-27B has a feedback thermistor, under very slow or no-flow conditions it can still overheat. This is because the thermistor is located near to the outflow end of the heater. As a result, this thermistor can cool slightly when the flow is off and that will drive the temperature up.

By comparison, the SF-28 has thermistors placed more strategically throughout the heater body and can maintain the correct temperature even under no-flow conditions.