What parts of 12-speed drivetrains are compatible between brands? Now that Shimano has released their own 12-speed MTB drivetrains (SLX and XT), I have the following question:

What parts of these drivetrains are compatible between Shimano and SRAM?

I would guess that chain, chainring and/or cassette (assuming wheels with the respective freehubs) would work with the rest of the drivetrain being made by the other brand while combining shifter and rear mech from different brands wouldn't work. 

Is this correct?
The key issues relating to compatibility is the width and spacing of the cassette sprockets (and chainrings), chain design and the design of the ramps and pins on the sprockets and rings that facilitate shifts. SRAM followed Shimano up to 11 speed but I believe they diverged at 12 speed.