I thought I would check this since I have all three holders. I checked the Polaroid 545 holder, the quickload, and the Readyload with two scrap sheets of Quickload (screwups in the filed I saved for this sort of thing). I noticed I had one warped piece of Astia, and one perfectly flat piece of Acros. The warped piece of Astia has wavy edges, and I'm not sure what the cause of these were. This may have been the film I was having trouble with years ago with the Polaroid holder. I had assumed it was the holder and not the film.
The polaroid holder uses springs to hold the edges of the film against the fixed back of the holder. This worked fine, and the only parts not perfectly in line with the rest were about 1/2 and inch on the end away from the clip, which was bent towards the lens by 1/4mm or less. Not terrible, but not great. This seems to be because the springs don't extend all the way to the end. The springs were not strong enough to make the wavy warped film lie flat. Not really an issue as most film shouldn't be warped.
The Readyload hold uses a pressure plate to pus the film into the from rails of the holder. This worked well for the flat film, but the spring strength wasn't enough to flatten out the warped one. So with my warped film you would have definite focus issues, but again not a real world issue.
The Quickload holder also uses a pressure plate, and the springs on this were much stronger. They were almost able to flatten the warped film out, and the focus issue would be much less.
In the end all three holders should do an equal job of holding the film flat and in the proper plane. The polaroid holder may show a very slight focus shift on one edge, but probably only a few mm of image are would be affected. I wouldn't worry about it. If I needed to buy a "new" holder I would probably go for a lightly used Kodak Readyload as it should be cheaper than the Fuji. Why not the Polaroid holder? Simple, it's heavier and the point of Quickload film is to shave off the pounds. If Polaroid film was still made, it would be a different story.
Another bonus of the Readyload holder is it came with a nice nylon pouch for protection.
Hope this helps someone.