Nested prefabs are truly great and i believe many of you have probably started using the new beta. I had a lot of trouble with crashes and instability, and even when i got rid off all the errors, warnings, unused scripts, bad references and so on, the problem persisted, so i thought the problem may be in Playmaker.
It isn't.
Set Scripting Runtime Version to .net 4.x and you're good to go.
Cheers