Any heat source can burn if it doesn't have any air flow around it. When you bury a heat mat, and the heat has no where to go, the heat concentrates in one spot and will burn out. I've burnt out a heatcord and a heat mat - both due to my own error. The green heatmats do have a bad reputation. Mine was underneath a few layers of newspaper and one day I found the bottom of my tank almost burnt right through. I was lucky my beautiful jungle boy didn't get burnt, or even cause a fire. The heat cord - I made a panel and on the underside I had foil sheet so when I mounted the panel on the wall, the heat would be forced back into the enclosure. But when my olive girl's belly swelled up after a feed I thought she wasn't getting enough heat so I put the heat panel on the floor and placed a piece of melamine on top of it. Ppl say the heatcords don't burn, but when placed like I had it, where is the air space? there wasn't any. I didn't touch the heat sources once they were in place so it wasn't until some time later I realised her tank was cold that I needed to check it. The heatcord melted onto the wire mesh I had it weaved around, it burnt the shade cloth and even the foil sheet was crazy cracked, and the underside of the melamine that was sitting on the cord, was black and cracked.
So keep this in mind when choosing your heat source. Some brands are better than others, but most importantly it is where you put it that makes a difference to safety.
I now have heatcord panels mounted on the walls of the enclosures, and an area built up with a small box leaning onto the cord, so the snakes can get close to it, and inside a hot box if they want to. It is safe, lots of air flowing past the panel so less or no risk of burning.