Does the ruling on sleeping in an unenclosed space differ according to individuals, customs, and the size of the place, or is it always reprehensible (makrooh)? And what would prevent it from being forbidden (haram)?

It is disliked to sleep on an unfenced rooftop because it exposes one's self to danger. This varies according to individuals, their customs, and the spaciousness of the rooftops. The prohibition may even reach the level of impermissibility if the danger is significant. The overarching principle in all of this is the preservation of life.