I served many vipassana retreats through SN Goenkas Meditation centers, people are dumped head first into their mind body sensations, many if not most are very detached from what's really going on in the mind and body and not ready for what may come up. Some go kinda crazy just from boredom, we all are entertained through the media now in days.
All sorts of repressed stuff boils up! The centers try and screen people to make sure they are stable but one never knows what's hiding in the dark corners of the mind until they shine a bright light so they can see. Many can't remember to view all sensation with equanimity, the 4 jhana stabilizes this but again this tradition doesn't focus on the jhanas..
Vipassana has sixteen nanas, the ones I found disconcerting like baya nana, the knowledge of the body as not self and not permanent, I felt as if I was dying but blissful after the fear dissolved. The patisankha nana, which is not much fun, one clearly sees anicca , anatta and especially dukkha are very sharp and distinct, they are after the bhanga nana which is dissolution of the body, there is nothing to focus on, scary but also very pleasant, the body just disappears. These nanas can cycle through really fast or a student can get stuck without proper support.
My tradition our primary sutta was the Satipatthana. Nana or insight which realizes this nature of mind and matter is called Namarupa-pariccheda-Nana. It is also called Ditthi-Visuddhi.
I'll be looking forward to your articles @surgo!