Unless they're conformally coated.

Why would they do such a thing? Conformal coating is good for harsh environments if the electronics are actually exposed to the environment (like liquids), but in a car that's not the case, it's protected in a box and usually put inside the cabin too.

Also, conformal coatings are very, very bad for heat transfer. Power MOSFETs usually need to be heatsinked; conformally coating them would work against that.