> It stems from the belief that our brain is dissociated from our mind/identity, and therefore existential-like issues should be treated as separate from the brain.
I have a "put on the brakes" mentality about hallucinogens, and it stems from a Christian worldview, where existential issues encompass both the brain and the mind/identity.
If anyone is interested in knowing, the reasoning about hallucinogens from a Christian worldview would probably go something like: (a) everything in life has a spiritual implication or connection; (b) we are created above all to know and love God; (c) knowing God comes above all through being grounded in truth and reality, centered in the word of God (scripture), prayer, and community; (d) while hallucinogens seem to have some positive effects on the brain, they also cause one to have intense personal experiences that are disconnected from reality, potentially opening oneself to negative or false spiritual influences; and therefore (e) the major risk of hallucinogens is that they can alter people's spiritual capacity to know and desire God.
As for using it to treat depression, anxiety, terminally diagnosed patients, etc. -- I would be the last to want to deprive distressed people of a straightforward cure. But I'd avoid recommending them anyhow: my fear would be that while hallucinogens might help temporarily (i.e. in this life), the effect in the next could be catastrophic because it may have contributed significantly to suppressing their innate human capacity to know God, find his love, grace, and forgiveness, and be satisfied in him. Meanwhile, there are other helpful ways to treat things like depression that God has given -- the beautiful natural world, for starters ("forest bathing" is a thing because it's a really good idea; modern cities contribute significantly to anxiety).