Radiant heaters do not heat the air, they are electromagnetic radiantion that has yet to be converted into heat. Once it reaches a physical material, the energy is then converted. Using such heaters in a grow environment would likely be not a good idea, you would not be warming the air and even if it is heating the plant itself, you have no idea how much. You could be cooking it or not providing enough heat.
I will let others detail their experiences.