See for me that would be my one hard no from UA (outside of some things where there is now an official version). I had enough trouble knowing how to DM for a whole new character class I had never played or been interested in when I let a player be a Blood Hunter. The UA for the Mystic is 28 pages long. Either the DM has to be super into it or the player has to be someone who is really good with rules and policing themselves, because the normal back and forth between DM and player that generally enforces class mechanics at every table I've been at just would not function with the Mystic.