Unity is a multi-threaded system. But the main UnityEngine APIs are not thread-safe so they can only be called from the main Unity thread.
I plan on examining the PlayFab source code but I also wanted to get input from others in case I miss something.
Do any of the PlayFab calls make calls to the UnityEngine? If so likely they cannot run on a thread other then the main thread.
The way I designed PlayFab data sync calls PlayFab C# will push it off to another thread. (A side effect I did not plan on.) I am trying to decide if I should redesign before I code or if I should push ahead. Thread collisions are really hard to test for so it might test ok but still be hosed from time to time not hosed in the editor but hosed on a mobile device :-)