I came across a problem where I can't pipe the material output from one node group into another node group (for mixing etc), it just appears as the default white material (see picture).
It works fine if I just connect a material node to the second node group.
Here's the setup:
A node group with a material setup.
Another node group with another material setup + a mix material (and texture to control it, just a checker texture in the test) to combine the material with an incoming material.
(The third Suzanne should be gold + red)
(I've also had problems with a node group, with a material setup, inside another node group [same result: it gets ignored], but that I couldn't replicate in my test file, but can't get to work in my real project. ETA: The problem appeared in my test file as well: If I [in a node group] connect the output [OutMat] of the group, instead of just a material node, it tries to refresh the viewport, but just stays at "Waiting for image", if I reload the viewport the material appears white)
Using the 17.5 TEST / 2018.1 RC 6 version.
(I've tried with Nodegraph optimization ON and OFF, with the same result)