Regarding your comment, I have explained below that, (i) if one approaches meditation mechanically, using a ‘do this to get this’ approach, or (ii) if one learns meditation with the aim of gaining more ‘hedonic happiness’ in one’s life - then it can be problematic. Before delving to that - regarding your statement:
This happened because the Buddha-to-be had not YET figured out the path to liberation. After figuring out the path - any so called “adverse effect” is seen as one’s ‘here & now’ (present moment) experience and such investigations lead to deeper insights into the workings of the mind.
Regarding the long list of modern day examples of ‘harm’ - please note that this type of list can be complied for ANY skill one might be learning. In terms of ‘physical issues’ that are listed, examples include: learning to cook, learning to skate, practising gymnastics, swimming, mountain climbing, etc. - the list can go on and on!
Regarding the listed ‘psychological factors,’ such as people with depression whose mindstates worsen, and/or people who are reliving past traumatic experiences: if this is happening, that means they are entertaining and ruminating on thoughts instead of observing thoughts (i.e., instead of observing the presence/absence of these thoughts and gently bringing the mind back to the meditation object). If such drifting happen, changing the meditation object (e.g. switching to loving kindness meditation, or doing walking meditation, etc) can be helpful. So, in other words, it is best to contemplate what remedies can be taken to address these “adverse effects” rather than researching and coming to the final conclusion that meditation is problematic to some people! So, just like how one might provide good instructions for developing any skill, for meditation, the advice one needs to provide people first is to clearly explain what exactly we are trying to do when meditating. Buddhist practices are NOT about cultivating hedonic happiness - it is ultimately about understanding the nature of human experience (although hedonic happiness might be a ‘side effect’ for some householders, because they are less bothered by and are able to ‘let go’ their day to day stressors). Therefore even when one experiences these types of so called “adverse effects”, the reaction should be ‘how do I overcome this’ - rather than thinking ‘Oh, meditation has all types of problematic issues, and therefore I should warn others!” All adverse effects are overcomable if someone has sufficient interest in understanding the deep teachings of the Buddha. If someone is not interested and is only meditating to gain hedonic happiness - then they might as well give up meditation or do only a little bit of it as needed!
Regarding ‘psychiatric diagnosis’: remember that this is a highly contested area! I am not sure if you know that the DSM disorder labels are “diagnosed” using check-lists – there are no objective tests (blood tests, tissue tests, X-rays, etc.), because decades of research have failed to find any structural or other brain differences between so called “psychiatric patients” and healthy individuals. I suggest reading some articles in the following website (psychiatrists and other qualified people intelligently discuss these issues here: Psychiatrists Call for Transition to Social Rather Than Biological Treatments
The Buddha beautifully described how our minds work, and understanding the teachings ‘within ourselves’ is what gradually leads to spiritual liberation. So, even if so called “adverse effects” arise in meditation, these point to how our minds work and this is how they need to be understood rather than thinking “oh, it is taking away my hedonic happiness!” For example, we get depressed due to our attachments (depression is a result of losing something we attach to - and these involve any of the five aggregates - this is explained in the following article: Theoretical Foundations to Guide Mindfulness Meditation: A Path to Wisdom | Current Psychology
(I think a pdf of this article is available via google scholar).
So, these “adverse effects” might even be able to pin point the areas we are attached to and remedial actions (in the form of meditation) can be taken by practicing different types of meditation practices listed in the Four Foundations of Mindfulness, and also ANY subjective thoughts can be observed as the here and now experience (panic, etc.)
Regarding strange behaviours that people display: this appears to be happening due to people taking psychiatric drugs, and suddenly stopping them (these are known as “withdrawal effects”). Here, we also need to know that pharmaceutical companies come up with psychoactive psychiatric drugs (using a trial and error approach) with a profit-driven motive and they come up with them without ANY theory regarding understandings related to the brain - see the following article: https://www.nejm.org/doi/full/10.1056/NEJMp1910603
See also the following: Depression: why drugs and electricity are not the answer - PubMed
So, when people suddenly stop taking these drugs - that can trigger what are known as ‘withdrawal effects’ - this happens because the body has got used to the drug and it is unable to cope with the cellular (physiological) effects that happen when the drug is no longer there in their system. This can result in bizarre behaviour - it does not happen due to meditation. Note that currently, a large proportion of people (~20%?) are taking psychiatric drugs and therefore this phenomenon is more widely seen than before.
In summary, Buddhist meditation is not about blindly following some specific system of meditation (i.e., ‘do this’ to ‘get this’ type of approach) - the reason for the meditations and the ultimate aims of meditation and mindfulness are beautifully described by the Buddha. So, if someone clearly understands the teachings, then they would not suffer from so called “adverse effects” - and even if they do occur, these experiences offer a window to understand the teachings at a much deeper level.