The heat from the miners are not for warning food. It is advisable not to try it at all. Warm your pizza with oven, microwave or air fryer.
Heat from miners don't produce any form of radiation which I might consider unsafe for food. Miners are thermodynamics machines and the heat it produces has no known health disadvantage, additionally it is not in direct contact with the pizza, heat reaches the pizza by conventional means from its pack. The heat from the miner is in the same category with that of your listed preference since all of them produces Electromagnetic fields (EMF) which comes from it's power supply, cables and it's circuitry.
I do not know if it is good for health or not. I am not also that interested in knowing it. It take just few minutes to warm Pizza if it is already cold. I think such things are done to create more views than something he will continuing using it to warm Pizza.
Radiations are generally used to make food clean from microorganisms. Processes sealed foods undergo radiation which kills the microorganisms in them so the sealed food can last longer. Even Microwave use radiation to warm food.
The microwaves do use an electromagnetic radiation but it non ionizing radiation, it not same type of radiation that is used to irradiate food for killing microorganisms. For that of miner here I do think the bigger concern is how the food is heated and hygiene.
This mining equipment can have dirt and dust around it and its heat may not warm up the pizza evenly, so even with the heat itself not being dangerous
using a microwave, air fryer or even oven is still much more better way to warm up food.
