The simplicity of the fresnel blend mode means that a number of physical effects are ignored.

From a physical viewpoint, this layer mode ignores the top layer for the incoming ray, and ignores refraction of the outgoing ray. The IOR of the top layer is also ignored by the base layer, meaning that the relative IOR is incorrect unless it has been manually set.

The rough coating layer mode extends this simple model by approximating the physical effects that it ignores.