Also are you in the talks with major texture providers such as Quixel, Poliigon. They both have plugins for importing textures and both don't work at the moment. Getting materials as pure glass or with 99 bump and so on.
We are in touch with them and reporting such issues regularly. I think the 99 bump issue is already fixed on their side and this should be available in the newest version of the exporter, or at least it is being worked on.
As to your previous report about the Archmodels 251 scene:
1) The appearance of the material changes (some features of the material become invisible):
For example, the cracks are becoming invisible - this is because they are fully refractive in the original material.
Please understand that if the original material was constructed in a highly un-physical / fake manner, then it may not be possible to recreate it with the Physical Material. The converter tries to keep similar appearance, but that is not always possible.
In this case it seems like keeping the Legacy Mtl may be the best solution.
2) Generating Mix nodes on top of Mix nodes:
This is actually related to point 1. The converter tries to keep similar appearance, and since the material is set up in a "tricky" way (using diffuse color, reflection map, glossiness map, IOR map, opacity map,....) for a simple asphalt material, it becomes problematic.
I am afraid this may be a case of "the converter cannot fix what was originally broken".
But still, we have this logged and will see if there is a way to keep better appearance and simpler shader networks (can't promise anything though).