Just as a point of order, in a separate discussion on on another board (not going to link) I ran the math on Mystics, with the best possible damage abilities, against an ideal target, and using their Psi points in the most rational way possible (Wu Jen 30' range weapon thing, as it also gets weapon damage), and I'm still coming up short against other classes, damage-wise (again even in simplistic scenarios which wildly favour the Mystic).
There doesn't seem to be any conventional scenario where they are really that dangerous/powerful.
They were really versatile, and re-reading I can see that. But only if they didn't spend all their Psi on damage. Most of the complaints I read about them work on the basis that they could both spend all their Psi on damage AND spend all their Psi on versatility. That literally could never happen.
So at this point, I think we can see WotC acted on a lot of really frankly wrong feedback, because it claimed Mystic were "too powerful" (something WotC repeated here, albeit they didn't say they shared that opinion). They literally are not, unless you run the 5MWD (at which point they totally, but so is every class with "daily" resources, especially Sorcerers and Wizards).
I didn't like Mystic very much. The fluff is ghastly and totally wrong. What the heck is Wu Jen even doing in there (it's also the source of the problems damage-wise!)? The whole thing is a bit of a mess, almost designed to fail, but "too powerful"? Absolutely not. Not if you're running 5-8 encounters/day, which apparently everything is balanced around.
I'm too lazy to re-do the math here but will help critique if someone else does. Basically, totally optimal play, where you're a Wu Jen and know exactly how many rounds of combat there will be in a day, and there are exactly the right number so you can use EVERY Psi point on combat, you still end up behind. Not by much, but behind.
If you do what people complain about - burst hard - you don't actually do very much damage, and you end up WAY behind over the day, instead of slightly behind. All the while you're acting like a Rogue re: AC/HP but you have none of the utility of a Rogue, because all you're doing is dumping Psi into wood weapon or whatever it's called. Take out that ONE ability (none of the others are close, damage-wise, because they don't get weapon damage) and suddenly Mystics are even further behind.
Mystic could have been fixed by re-fluffing, removing or nerfing a few abilities, and restricting versatility by limiting which disciplines you could have chosen in a harsher way.
I'm not saying it should have been, but it could have been, and they didn't bother to try, seemingly on the basis of at least partially spurious feedback. This is the dark side of the 7/10 thing if that's really how they work. If enough people (say, 3.1/10 people) have a false belief about a class/subclass/spell/feat/etc. it doesn't matter that it's false, because it'll fail the 7/10 test.