From what I know, mystic extracts are gained only by leveling up mystic enchanting. Leveling get increasingly harder.
This mean the player (me) has to be careful which enchant to extract or not. Furthermore, the level is being shared account wide.
I like to try out some different builds for enchants (like we do with talents), and not spend 10h building my character, because of limited extracts.
It is not even possible to purchase mystic extracts in the shop.
It is punitive, the same as with early Diablo II preventing you from changing your talents.
I'm lvl 87 right now, and leveling up is boring, long, and unpleasant (= rerolling good enchants that I could sell at AH). I don't want to grind extracts nor do I want to spend 10h planning ahead, I just want to play casually.
It also feel contradictory with the design intention of free talents pick, builds exploration of Ascension server area52.
This is enough for me to stop playing.
*Just to be clear: Each time I use an extract, I may be shooting myself in the foot, and locking myself forever in a grind loop or wrong builds. Not a good feeling.