In this Sandy of Cthulhu video on YouTube, Sandy proposes that horror is a delicate genre. When you try to combine genres, you typically end up switching to the other genre. He gives examples of horror/adventure or horror/kaiju combinations.
Then, using comparison and contrasts, he shows the exception to this, which is when you combine horror and comedy. Find out how movie producers are able to interject humor, while still retaining the integrity of horror elements.