If this is such a common problem, where are the dark-nights-of-the soul amongst monastics in the Suttas (and not the ones associated with asubha)? Nearly everyone is described as having a relatively radiant expression, being peaceful of comportment, and so on. There are descriptions of there being benefit to the Path even for those with tears streaming down their face, so there is challenge, but this is not easily associated with bhavana.
Samadhi is soft, and not harsh as here, due to sila, brahmavihara, samatha, jhana. There are obstacles to
the Path, but for those experiencing dark nights as part of
the path, I wonder about the place and degree of these things for them.
This also serves as a response to 'why meditate' - it can be imagined ("meditation is like...") as hard-wiring kusala
("!") improvements (sila, et al
) and trimming akusala
counterparts. This prevents dark nights as every insight is pleasant confirmation of the Dhamma ("...revolves around right view", preventing confusion and fear in the face of knowledge, insight), not stressful contradiction with expectation & habit.