The Principled Shader (PS) correctly applies Fresnel all the time. Brilliant! But is it based on geometry or on normals? If I have a high-poly figure with many facets smoothly curving away from the camera, I get the correct Fresnel at the point facing the camera and the point orthogonal to the camera, and all points in between, right? What if I use that high-poly to bake a normal map for a low-poly version? The geometry won't have much of a continuum of angles, but the normals will.
I think the answer is normals -- the Shade Smooth operation is just interpolating normals and I'm certain the Fresnel is calculated correctly. Can anyone confirm how this works? Or if there is a cutoff, perhaps, where you need to have a minimum amount of geometry for the Fresnel to work with the PS?