Well, I've shot a half dozen architectural projects with the setup recently and I haven't seen any distortion issues. As for fall-off, remember, these lenses are designed to evenly illuminate a 6x6cm piece of film, so yeah that hasn't really been a problem either. They tend to fall off dramatically at the limits of the image circle. I'm only interested in real world results and haven't looked at any of the graphs, but after extensive use on long shoots, I'm pretty damn pleased with this rig.
About the only thing I'm questioning at this point is whether I want to go back to my M-Line 2... smoother and more robust than the Universalis, but heavier.