Even if psionic disciplines are different from spells (as psionic is different from magic in general), there seems to be a clear correspondence between them, as well as between psionic talents and cantrips, because in some way things must get balanced.
Spells have an explicit limitation when it comes to how many of them you can cast in a turn. If you cast a spell that requires a bonus action, you can then use your action only to cast a cantrip.
I haven't seen a similar limitation for psionic disciplines. Did I miss it somewhere? If not, have the developers said why psionics doesn't have that restriction?
Answers regarding developer intent must include quotes from the devs. Speculation is not acceptable.
Best Answer
As long as the action economy and psi limit allow it, yes
We see on page three of the Mystic class description:
Based on this, it would be inappropriate to use the standard spellcasting rules when using disciplines. Additionally, page 9 states:
As you have noted, nowhere else in the class description mentions a limitation on the number of disciplines that you can cast in a turn, and we shouldn't assume that any rule carries over from regular spellcasting. Based on that, the most specific rules that we have is the standard action economy rules: you can only take 1 action and bonus action per turn, and are limited to 1 reaction a round.
Additionally, Jeremy Crawford has been asked this very question on twitter and he replied:
Since Disciplines are not spells, they are not held to the same limitation. As long as you have an action/bonus action/reaction left to use a discipline and the proper number of psi-points, you can continue to use your disciplines as you see fit.
Keep in mind your psi-limit though
The mystic does have the limitation that:
This doesn't prevent you from using multiple different disciplines in one turn, but it does limit the way that you can use your disciplines in a manner similar to the spell slot levels that a typical spellcaster has.
Have the developers said why disciplines aren't restricted like spells?
Sort of. Often, as with the Kensai Monk, the Unearthed Arcana material intentionally experiments with unusual features, for example not immediately making Kensai Weapons also Monk Weapons. They could be performing a similar experiment here. The spirit of this material is that the DM will make a call regarding ambiguity in the class, and then the surveys will inform the team about how people are playing the class. Or as JC put it here:
Based on this, I would say that their decision not to have the mystic follow the same rules is intentional.