Is not Spiritual Realization the experience of oneness?
That notion is one common reason why some seekers seek. In essence, it is not unlike the reason the addict keeps returning to his/her habit. We got a high and we want the high again.
Someone along the way decided that enlightenment is equal to permanent pleasure; and the pleasure here happens to be a higher state of consciousness.
Given the amount of psychological suffering we can experience the idea of having only the positive half of the universe and discarding the other half can be appealing. But some focussed introspection reveals that to be a pretty naive thing to believe in and pursue. Nevertheless it seems we all fall for it at one time or another in our lives.
Whether it is through LSD, Ayahuasca or advanced meditation practices, makes no difference as far as the end of suffering is concerned. Although those practices can aid in ridding the personality of useless features, which is important, they have the potential of creating fascination and taking the seeker on a detour.
The experience of oneness is part of duality therefore it will eventually lead to the opposite experience. The yogi must come down from his cloud and eat dinner and wash the dishes and feel like an animal amidst other animals again. That is what duality is about. Fight it and you will suffer more.
Of course, there are stories of yogis that are so skilled that they become completely unconscious of the physical reality and stay in bliss. Wonderful! Then they have to eat and go to the bathroom like the rest of us and the bliss that came through meditation is lost. So they go and sit in meditation again to find the enjoyment they were seeking.
The Oneness of spiritual realization is not any oneness that can be experienced and lost and then recovered. It is a quiet dwelling in the normality of awareness which pervades everything and is the source of everything. There is no experience of loss of oneness, and there is not experience of having it either, because the imagination of separation was removed.
As for the end of suffering while being grounded in the reality of the present moment, it is not related to permanent pleasure and exclusion of pain. The end of suffering is the total and unconditional embracing of duality. With the caveat that there is no one to embrace it. Total peace and tranquility; unconcerned and uncontrived.