Hi,
It's going to be about compatibility issues with 5.6, and core improvements, I don't think there are major new features atm. Playmaker is in active beta testing and should be release around the same time as the 5.6 public release.
There is a known bug when you port a project from previous versions of Unity to 5.6 that looses references to PlayMaker components, it's been reported to Unity so hopefully the public release will have that fixed.
Bye,
Jean