Kajiya-Kay Model seems to be not realistic enough compared to Marschner model. The hair is so too bright, glossy, and lack of details.
To my understanding, the reason why the Kajiya-Kay Model renders hair to be so glossy and "perfect" is because it does not consider the texture irregularities that exist on the surface of the hair shaft. This would cause the light that hits the hair to bounce with less variance in direction, concentrating light to create this overly shiny appearance. The Marschner model actually models the irregularities of the hair shaft, scattering light in a more variable and realistic way.