“Write what you know” has been the mantra of anyone I ever asked for advice (and some who I never really approached) about my writing. My father, my teachers, even professional-opinion-havers like people who actually get monies to write/edit/judge literature agreed: Write what you know. Why? Because… well… it’ll seem more honest. There will be a depth and sincerity to your prose otherwise lacking in a five hundred page opus of space monkeys and undead machines. Contrariwise, you can give your supernatural epic some sort of grounded human theme like loss, greed, or the great topics of the day (like is it ok to disagree with the President even though he’s black?).
Real heady stuff.