Major problem with flat roof--they can collapse from water weight when the drains become clogged with debris. Saw this first-hand in Stuart FL last year. Eleven condo buildings condemned in one complex when the flat roofs collapsed.
Not much danger of collapse from a bunker/saferoom flat roof. It would be a lot smaller and have steel bars connecting it to the side walls before the concrete is poured. Not really all that expensive but not much danger of a collapse either.