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. You can ultimately end up with David Koresh. Further on this point, mysticism can lead to intolerance. Anything worthwhile that a mystic has said, has been said better by a philosopher. This is true because philosophers do not appeal to private experience.