If you see this warning when you render
Warning: [mtoa] [translator polymesh] ShadingGroup exampleSG has no surfaceShader input
it’s because a shadingEngine node either has nothing connected to the surfaceShader port, or (more likely) it has an unknown node plugged into surfaceShader.
For example, if you load a scene that was set up with a different renderer, and that renderer is not loaded, then you’ll get some “ShadingGroup has no surfaceShader input” warnings.