There are no imposed limits on the complexity of the exported meshes, but this looks like a bug or a misinterpretation of the data sent by XSI to the plug-in (ok, that's another name for "bug" :) ).
Of course, we'll only use the scene for debugging purposes and we'll delete it once we're done investigating. We can sign NDAs and such if needed.
We can add code to approximate the mental ray architectural materials but it probably won't be a perfect match.
Please check Yog's last post in this thread
, maybe you're experiencing the same problem when trying to render.
PS: NURBS don't work at the moment, only polygons and SubD can be exported.