I'm also opening psionics to players in my new campaign, but since we meet only monthly and no one has expressed interested in a psionic class, I don't know if I'll have useful information for you in time for your campaign creation.
Maybe you can try it out in your campaign and let us know? (Someone has to take the plunge.)
The player who created a mystic with the second playtest version, felt it was too powerful and we changed his character to wizard.
There are just too many ways to be the character with the solution all the time without too many disadvantages.
Can you make a balanced character that won't overshadow everyone in almost all three pillars? Yes, but it is too easy to make a powerful master of all trades.
The psion character is fun to play with lots of interesting powers. Needs more balance.
I have played the Psion and found that the abilities were very good outside of combat, but required some clever uses. So I would call that balanced in my experience. I'm going by memory, but I think I had an ability to track an item and look from its perspective. The Rogue-Thief did a spy movie style swap of one of the badguys items with an identical one that I was bound to. It let us spy on the bad guy and unravel most of his plans. The level of scrying was far and away superior to anything another caster could do at that level, but it required tricks to pull off and it was kind of his main schtick.
I felt that the mind control tools I had were good, but I lacked anything like the versatility of a wizard, even one heavily specialised in charms.
I DM'd for a guy who was playing a 4e Battlemind made up in 5e using those rules. It worked very well. He was basically a slightly better Fighter, who could not sustain his power for as long as a Fighter could. In short fights he was superior, in longer ones a Battlemaster would overshadow him. A Fighter does not require the expenditure of limited resources to be good, the Psion-Warrior did.