Mysticism exists as a primary characteristic in Hinduism and Buddhism. Mysticism in the 'Abrahamic' religions, Judaism, Christianity and Islam, exists as orthodox expressions of religiousity but it is closely monitored by the ecclesiastics of these faiths.
2. Medical research is revealing that ecstatic experience and mental illness are linked. Perhaps many of the great mystics in history were simply manic-depressives. They may not have really seen God, but merely experienced a pleasant delusion caused by their psychotic state.
Mysticism which places spiritual reality as higher than mundane reality or as true reality must be separated from the mystical experience in which the spiritual and the non-spiritual are interdependent realities. Attempts to live in one realm to the exclusion of the other leads to materialism or a perverse form of mysticism, separated from reality and ultimately self-destructive.