WRT film flatness when using the Polaroid holder. It is true, but depending on you use it may or may not be a concern.
I've used my Polaroid holder with Ready Loads and Quick Loads for years and never noticed a flatness/sharpness problem. It wasn't until I used it to shoot an archetictural subject with strong vertical lines that I noticed a slight problem.
The lines were not perfectly straight, they had a slight curvature in the center, where the film bowed. Everything is sharp, but the film wasn't truly flat.
I rarely shoot wide open, so at f22 there's sufficient depth of sharpness to produce a sharp neg/tranny even though the film isn't perfectly flat. Whether this is a concer to you, depends on your subjects and shooting style.