It leads to speculation about fanciful stories that have no hope of corroboration. They are therefore divisive and destructive to all normal functioning. Studying this is anti-social. Mysticism adds nothing to the social world. This solitary nature can lead to quietism.
Mystics do bad philosophy.
There are no arguments, rarely any reference to the philosophical tradition. Furthermore, mystics may only have a rationality within their own system, but they are often quick to use rationality to condemn others. There is no ultimate value in the mystical insights, apart from having the experience. Mysticism makes for bad theology. This can lead to a situation in which the mystic can abuse the experience, gather power for him- or herself.