I rarely cover Unearthed Arcana content. The primary reason for this is that I simply don’t have the time. UA content makes it to final publication less than half of the time, and my crystal bar isn’t clear enough to predict what’s going to make it to print, so I’d rather spend that time on stuff that’s already in books and that I know everyone can actually use.
The Mystic is a weird case. WotC tried a few times to get psionics right before we got the psionic subclasses in Tasha’s Cauldron of Everything, and the most iconic example was the Mystic. It was the first full class that we saw outside of the core rules (yes, it predates the Artificer), and a lot of people are very excited about the Mystic several years later. I occasionally get requests from people asking me to write a class handbook for the Mystic.
To reiterate: I will not cover the 5e Mystic class in its current form.
Now I’d like to explain why.
For people who have not read the Mystic UA article: it’s 28 pages and includes six subclasses. That alone is a terrifying prospect as a playtest document. I believe that’s a larger page count than any other class except possibly the Cleric (there are lot of domains) if you total up the page count across every published source book. This thing is massive. And goodness, it is a mess.
The Mystic is riddled with abuse cases. The class is way too complicated, having more build decision points than the Warlock and more decision points during play than any existing character because of the way psionics and psi points work. Some features are just outright numerically unfair (you can get a cantrip-equivalent feature which is just Mind Spike with d10 damage dice instead of d6), and many allow unfair combinations that immediately outshine other characters.
This thing is not balanced. It’s not even close. I can fix Peace Domain with some patches and house rules, but the Mystic is too far gone. If I tried to patch it, it would be more patch than content. We’re talking a Ship of Theseus scenario here.
So you might reasonably think “Okay, what if I do an all-mystic party and just lean into the imbalance?”. That might work. There are a couple things in the class that will make you regret that like Order of the Avatar’s Avatar of Healing which provides an ability similar to Life Domain’s Disciple of Life feature, except that it also applies when any of the Mystic’s nearby allies regain hp from a Psionic Discipline, so basically everyone in the party gets Disciple of Life.
I can’t recommend playing the Mystic. I certainly don’t recommend allowing it at your table.