Those types of heaters are safe if adequate ventilation is provided.
Two-level ventilation is important.
An example is the ventilation used on Arctic Oven Tents. They have one opening to the outside on the lower level of the tent and another at a higher level at the other end.
Good cross-ventilation provided by the difference in the levels. Each of the two openings are about 6" in diameter.
And they suggest using a wood-burning stove. Other videos showed Big Buddy, etc heaters.
Safe if VENTILATED properly and secure enough so they don't fall down and go BOOM.