Well I thought mine would be a unique and entertaining anecdote. It didn't take much research to find many similar tales of Marineland Stealth heaters failing and exploding. As it turns out, it may have had nothing to do with it being under the gravel (though I still wouldn't recommend it). A potential manufacturer defect may have made the failure inevitable. I'm just glad I didn't lose any livestock or other equipment.
I live in Texas. In Winter, the thermostat is set to 60 degrees during the day, 70 at night. In summer, it's 75 at night and 85 during the day. In any event, my tank heaters aren't asked to work too hard (set at 78). On smaller tanks, I have more trouble keeping the water cool.