While looking into D2677, I just noticed an obvious issue in the current master (b94a433ca34, to be precise):
Under certain settings, the results of Branched PT and regular PT will be completely different.
For example, take a default scene and set the cube to use a Principled BSDF with MultiGGX and Specular=Roughness=1, then render with BPT and PT. The BPT version will be more than twice as bright.
I'll look into this, I just thought I'd formally report it as a bug to make sure that this is known considering how close 2.79 is.