Thanks for reporting these issues. Generally posting bug reports in a thread like this is fine, we pay close attention to the forum so we shouldn't miss anything, but I'll ask the team about adding a sticky thread.
I'll look into these issues but I wonder if you can recall the libraries that are loading V-Ray materials? I've looked through the Mental Ray mat lib and all the materials appear to be using Arch and Design shaders. I hope you don't mind me asking a couple of additional questions to help isolate the problem: When the issue occurs is the Material Selector set to Automatic? Does setting it to Mental Ray make a difference?
Some of the library presets use operators that are not compatible with Mental Ray. In the case of the wooden fences the Material operator used to switch between new and aged materials is not supported . It's worth mentioning here too that Segment->Deform->Mapping is also not compatible, you can read more about this here
but unfortunately this is a limitation of the render engines. Turning off Display->Render->Use Geometry Shader
will force these nodes to work correctly but the RailClone object will render as a single mesh with no instancing. If the polycount isn't huge this shouldn't be a problem. In the event that the operator isn't used we should make the clean textures the default as I imagine that would be more useful for Mental Ray users than the aged look. I'll change this for the next update.
Thanks for bringing this to our attention.