I offer the following, but it is not meant as a comment on any individual's personal practice or path or training recommendations. It is just a general comment.
Zazen and all meditation comes in many flavors and intensities. There is also much discussion lately of "Dark Night" syndrome, and Dr. Willoughby Britton's research on that ...
https://www.theatlantic.com/health/arch ... ls/372766/
My feeling is that certain very intense and concentrated forms of Zazen and other meditation are apt, for the particularly sensitive or anyone at a certain point, to push people over the edge into all manner of breaks, depression and the like. I also believe that Shikantaza (at least as I guide it, as a low pressure, letting be and easy slipping into Wholeness and Kensho which has no "into") is less likely to do so, but even here one must be cautious. It is much like someone coming for swimming lessons at the community pool who looks particularly overweight or who may have a bad heart. I welcome them, and believe all may benefit, but I have sometimes required a medical doctor or mental health professional's approval after interviewing them.
Our Sangha, because of its heavily "online" format, has a relatively high number of members with serious disabilities and health conditions. I have folks who crawl to the toilet because of pain and muscle weakness or are confined to wheelchairs, others with serious back injuries or panic disorders. I hesitate about recommending any standard postures or time limits to such folks, because that is a kind of "shaming" or "you can't do" message that may be conveyed to such folks. I teach a particular lesson about Zazen in which Zazen is -not- a matter of time or quantity at all ... or even about "sitting" for the person who truly realizes that truth. No "butts" about it. In other words, sitting for a moment or endless Kalpa is all the same, as is standing on one's head or crawling to the toilet. However we sit for a certain number of minutes in order to realize that timeless truth that it is not about time or sitting. For members with physical restrictions, if they can only manage a few minutes or seconds while in their wheelchair or reclining or moaning and writhing in pain on the way to the toilet, that is "good Zazen" and holds the whole universe.
I also feel that pop-psychology, new age beliefs, certain kinds of magic, soothsaying and mysticism, certain pseudo-scientific and folk medicine beliefs and the like which sometimes get mixed into Zen practice can frequently be harmful at worst, or simply distracting from the real power of our Zazen practices. (Of course, views and definitions of each of those vary by practitioner.)
Finally, it is just my personal take, but I believe that any type of meditation which leaves the student thinking that there is something to attain, even subtly, is harmful. Also, leaving them with the feeling that they can just stay wallowing as they are, in their excess desire, anger and divided thinking is harmful. One must thread the needle of getting the message across that one becomes free of excess desire, anger and divided thinking by radically releasing all thought of something needed or to attain. However, this is just my usual Shikantaza spiel, and I do not mean it as a comment in any way on what any other way may be teaching or not teaching, or to imply that only Shikantaza does this. It is only "harmful" within the context of Shikantaza.