I guess very little downsides, but for completeness' sake I guess they can be separated in categories where some of the "risk" is likely not to even be significant or realistic at very low supplemental doses meant for e.g. cluster headache prevention. On the other hand acute risk can be very different from use with chronic administration.
- Unless I am mistaken, occasionally mushrooms can produce headaches rather than cure them?
- Orthostatic hypotension (seems unlikely at low supplemental doses)
- Tentative: 5HT2B agonism not being healthy - unfortunately psilocybin seems relatively selective not for the psychedelic 5HT2A but the 2B, which is implicated in cardiac fibrosis but only with chronic use (so supplementing). One of the worst selectivities of most psychedelics apparently. But it is extremely hard to say how to interpret what safe levels are for daily use.
- At fully psychedelic doses of course there are risks involved simply for being a psychedelic. Many here will call the risks low and fair, and it helps to screen yourself for predisposition to mental issues etc. Regarding this: arguably psychedelics would just be a trigger for unstable people, and it could just as well be something different to tip a person over the edge... but when it does go wrong it can have a huge impact on your life. I have known several people who found psychedelics revealing but also got in way over their head with the awareness and insights, making life more complicated for years. I guess this is more like symptoms of personal crisis as a whole, I would not blame psychedelics for just fucking a person up out of the blue... but some mental processes catalyzed can be harmful, as a brain in overdrive can run equally hard in reverse as in forward mode.
At psychedelic doses, mushrooms / psilocybin has been successfully used for outlook in terminal patients by Johns Hopkins I believe? And relief from depression can be a psychological effect from the experience that may not have much to do with direct pharmacological action... so it is questionable whether low dose supplements can help, if they don't produce a trip. Don't know of the mechanism by which it would directly relieve anxiety or depression although I guess it is possible. If anxiety can be relieved even at low doses, by inducing a more meditative state of mind, anti-depressant action may be an indirect result of that?
I thoroughly respect shrooms, and they used to be my main psychedelic when I had little access to and experience with other ones... but honestly they give me the worst mindfuck-y confusing intoxication and emotional rollercoaster of any psychedelic, so for me personally it has huge downsides and I lost almost complete interest in them and would much rather choose 4-HO-MET... However some people have what I get with LSD instead of mushrooms, so completely the reverse... just like some people react well to Ritalin for ADHD and not Dexamph while for others its exactly the reverse.
I hold psychedelics in high regard, but as for benefits or downsides of a psychedelic, it really depends on which one and how you would use it exactly, and what for. It would be much too simple to say that they are just ideal.