I think there is nothing wrong in mysticism, " A belief in the existence of realities beyond perceptual or intellectual apprehension that are central to being and directly accessible by subjective experience", Buddha was a Mystic. I look at "Mysticism" as true insight. However, a mystic experience is only an answer, one cannot rely on Mysticism for food shelter or clothing. This brings me to the 10 Oxhearding Pictures. These pictures show a journey from begining to end, with a mystic experience in the middle. Korean Zen Master Seun Sahn, refered to it as the compas of Zen.
When we start our quest, wer'e after something more than we already have (enlightenment). As our journey progresses, we have a glimpse of the mind, 1st pic. Finding the tracks, we progress Through to seeing the ox, right through to no ox no I. Eventually though, we return with gift bestowing hands, 10th pic. This last pic. is returning to the start, or in the Zen compass, travelling 360degrees. However now we have real insight. We still have to work, cook and converse with each other , though now through this mystical insight, we know why!
The reason I believe the Buddha to be a mystic is: The life of the Buddha states, that he sat under the Bodhi tree untill he had the answers for which he was searching (enlightenment). This was a mystical experience.
If one has not achieved (attained) Enlightenment, how would one know if a mystic experience is valid or not, otherwise it's pure conjecture.