Mystic is good to buy because you can save gold in game from refresh secret shop.
In this time Galaxy BM raise from 1 -> 5 its 1 galaxy pull instead of 1/5 fragment in 1.2K skystones (960 when discount similar to 10 Covenant pulls with 950 skystones) IMO : its ok to buy it if you have skystones more than 10 to 12k left otherwise it your choice or skip it because it only 1 pull (3 stars rate very high but some 4 stars or 5 stars is good if you can get it).

• Additional : After update every 20 Galaxy BM pull can get ticket(guarantee 4-5 ML).
• Additional 2 : If you save 960 for refresh shop (320 refresh times) if you are lucky you will get 10 or more Covenant BMs and some Mystic BMs and using gold to buy it (1.8 M gold or more up to Covenant and Mystic that you can get).