Mystical union" is quite a theist concept.
No it's not, esp. as steveb1 has defined it (my first question was "what is mystical?" and then he defined his terms).
The problem is that as deluded beings we are
subject to ingrained habits of delusion and ignorance, in particular dualism. On a deep emotional level we persist in dualism and this gives rise to all the gross habits of ignorance: anger, desire, jealousy, attachment, pride, etc.
The problem is that most of us do not experience union or constant identification with Buddhanature however our specific school teaches this.
However the question was raised in the specific context of Shin Buddhism which presents a unique view and in comparison to other Mahayana schools seems to emphasize the duality between foolish, ordinary sentient being and Amitabha. In this context the experience of shinjin appears to the the ideal. The stories of people who have attained shinjin might be a good starting point.
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche