Burners are former Glowing Ones of which the radioactive materials within their body have decayed into stable metals, giving them their darkened skin tone. Nuclear fission-based chain reactions continue to occur in their base of the skull, heating the lighter metal particles in the jugular artery to the point where the blood glows orange with the intensity of the heat.
Burners have less HP than most feral ghouls due to their old age, but the chain reaction occurring inside their skulls presents both a tactical situation. When a Burner's head is pierced the nuclear reaction will be exposed, pushing out the molten metals at high force in a damaging explosion.
This can be used to the PC's advantage; if a Burner is surrounded by other enemies the head can be sniped, causing damage to everything in the blast radius. The likelihood of an explosion is heightened with the Bloody Mess perk. However if a Burner gets close to the PC they will have to be killed without exploding the head: It is advisable should you find yourself with a Burner in front of you to aim for the torso or use a slicing weapon, such as a laser or sword.