Never heard of one causing a fire. They are merely small heat-generators that lower the RH by raising the temp ... a low-wattage incandescent light bulb would serve the same purpose in addition to providing a bit of light. ;)
I think that the primary value of the heater sticks is by slightly raising the interior-materials temp over that of the exterior so that when you open the door and the outside air enters, the moisture in that air does not condense on the metal surfaces inside.
I have my gunsafes outfitted with one/each heater stick, Eva-Dry DH and small desk fan to keep the air constantly moving inside the box (the fans also add a bit of heat).