There are different forms and extremes of mysticism that have given the word a bad name.
Union with Christ is the central truth of the whole doctrine of salvation. Two become one, yet are still two distinct persons.
Union with Christ is Christian mysticism. Nothing more, nothing less.
"The wide range of similitude used in Scripture to illustrate union with Christ is very striking. On the highest level of being it is comopared to the union which exists between the persons of the trinity in the Godhead. This is staggering, but it is the case (John 14:23; 17:21-23). On the lowest level it is compared to the relation that exists between the stones of a building and the chief corner stone (Eph. 2:19-22; I Pet. 2:4,5). In between these two limits there is a variety of similitude drawn from different levels of being and relationship. It is compared to the union that existed between Adam and all of posterity (Rom. 5:12-19; I Cor. 15:19-49). It is compared to the union that exists between man and wife (Eph. 5:22-33; cf. Jn. 3:29). It is compared to the union that exists between the head and other members in the human body (Eph. 4:15,16). It is compared to the relation of the vine and the branches (John 15). ...The mode, nature, and kind of union differ in the different cases. There is similitude but not identity. (John Murray, Redemption: Accomplished and Applied. pg. 167,168)
Spiritual union and communion with Christ is Christian mysticism, no more, no less.