you know, I've had problems with almost every heater I've owned, including the little hydor, which really is the smallest in tank, adjustable heater I've found... EXCEPT the hydor inline eth heaters. I've had problems with every brand, make and model I've tried, except that one unit. I love it.
unfortunately, they're rather large and aren't great for a 5.5g Terrarium. lol.
something else you could think about...
I've considered making a terrarium with a glued in back wall, about 2-3" from the actual back... and then that area is filled with the water used for the tank, whether it be misting, trickling, or a water feature. The reservoir can hold a heater, pump, etc, generating radiant heat and humidity as well as serving as the water source.