Not all is good news. Here in Classroom scene it introduces much more noise with Light Tree enabled:
Light Tree OFF:
Light Tree ON:
(100 samples, 50% original size)
So I’m not sure why that happens.
Edit:
Apparently something strange happens with Area Lamp called “exterior_fillLight” outside the window. So maybe the problem is simply something that is not yet supported.
Edit 2:
Looking for GSoC reports, I’ve seen that Erik knows about Classroom problems:
https://lists.blender.org/pipermail/soc-2018-dev/2018-August/000169.html