After messing around with Color Efx Pro 4from DxO's Nik Collection 3, I think my criticism and reaction to the SOOC JPEGs from my Fujifilm Film Simulation Challenge Roll 5: Kodak Portra 400 test was "user error". I think the exposure compensation dial may have moved during my photo walk. That would certainly explain the washed-out look of the SOOC image. As a test, I used Fuji X RAW Studio to reprocess the images with Ritchie's Kodak Portra 400 recipe and adjusted exposure compensation settings and the results are much better. I think the photos were over-exposed by between EV +1/3 to +1. In the interest of transparency, I left the original blog post but included reprocessed images at the bottom.
I then tried the Kodak Portra 400 film presets from DxO Color Efx Pro 4. DxO Color Efx Pro 4 produced very different images than the SOOC JPEGs created by Ritchie's Kodak Portra 400 recipe for X-Trans III sensors. The images created by DxO Color Efx Pro 4 are consistent with what I had expected based on what I had seen and read about Kodak Portra 400.
I don't know for sure why the two approaches are so different but even in the age of film, the process for developing, the paper used, etc. were all contributors to how a print would be rendered. I'll just have to shoot another "roll" of simulated Kodak Portra 400 and compare results again.
Although I don't have an X-Trans IV camera, I am going to try [Ritchie's newer Portra 400 recipe].