In the discussions about trauma, anxiety, and depression (The Precision of Sensory Evidence and Coherence Therapy) there was reference to using meditation, psychedelics, and somatic therapies - other alternative therapies were mentioned in Astral Codex Ten links as well - for their ability to smooth out the mountains between different parts of brain function, which don't always talk to each other well. These are often accompanied by feelings of general benevolence, a sense of being one with the universe, a belief that one has achieved a more distant perspective that is a truer reality than the everyday anxieties one is living under. One knows oneself to be small in the great scheme of things, yet is satisfied with that.
It would be easy to mock this, and many have. If you can get this same deep philosophical insight from a microdose of LSD or any of a half-dozen types of massage as you can from meditation, doesn't that make it all rather hollow and false, then? I think that is a strong point but I should be cautious about drawing that much conclusion from it. Our bodies and emotions have a very limited suite of responses that is greatly moderated and interpreted by the brain. Some pains and pleasures seem inseparable, there are workshops which teach you to reinterpret anxiety as excitement. A philosophical insight would be as unlikely to discover an entirely new sensation as a tour of an art museum reveal to the world a new color previously unknown. Our highest selves are assembled from bits of the lower.
Nonetheless, humility might be in order for those who claim insights. If it is no shame that such feelings and impressions accompany the experience, neither can they be regarded as evidence of truth. Such claims will have to derive from elsewhere. The overwhelming feeling will tell you that it is its own proof, and those who don't see this lack your understanding (and are thus unenlightened).
The same sort of complaint is sometimes leveled at Christian experiences, of people saying So all that happened is you sang a lot with other people and felt really peaceful, but you are calling it God. Many Christians do fall into that same somewhat arrogant claim of superior understanding on the basis of experiences that can be created in other ways that have nothing to do with the faith. The claims of truth have to be founded on that foot that is still placed in this world, not some other. I believe the Church makes those claims for good reason despite the large elements of woo - at least to appearance - that can occur in worship and are even encouraged in the Scriptures. The most important part of prayer might ultimately be our facing God, but we are firmly instructed not to cease praying for our daily bread, for the sick, and for strength in trials. Those functions are kept more separate in Hinduism, where the pagan celebration and the ascetic mystic are both part of the religion, but not together.
It is for this reason that I view the alternative treatments as quite legitimate to fix something, to open some drain that has been clogged or reconcile some communication between parts that has been scrambled. The feeling of oneness may derive not from being accurately connected to the universe, but from the various parts of the brain that have become sundered becoming one, or at least finally in contact with each other. I am more suspicious of them for everyday use. We might need to be fixed many times or tuned up repeatedly, but it also may be that once the treatment is completed we are now simply seeking the feeling.