Up front bias disclosure: My company (Digital Transitions) chooses to sell cameras (Phase One) that do not do pixel shifting and chooses not to sell cameras (e.g. Sinar, Hasselblad, Fuji) that do. So I'm obviously biased. But I also have quite a lot of experience working with clients to evaluate against these options.
On Color Accuracy: Multishot does not improve color accuracy*
Resolution: Multishot can improve resolution but the subject and camera must be perfectly static, and the lens and aperture must be able to resolve the subject at the small aperture values required. The severity of those constraints increase with the resolution being discussed. For 50mp sensors it was already very challenging to use pixel shifting successfully. For 100mp sensors those constraints are very very considerable.
Aliasing: Multishot will almost always reduce aliasing (which is good), but can also introduce significant pixel-level artifacts that can be difficult to detect without reviewing every image at every part of that image at 100%.
If using multishot in a workflow environment (e.g. trying to get a certain amount of work done in a certain amount of time) it's also notable that multishot captures requires more capture time, more processing time, more pops of the flash (if using strobe), more shutter actuations (if using a mechanical shutter), reduces raw compatibility, and reduces the number of viable shooting environments. And while it does not explicitly prevent you from using other techniques like focus stacking, exposure bracketing, frame averaging, or stitching, it does significantly increase the number of frames required to accomplish them.
I'm biased, so I get that my opinion here is of limited value. But I would very strongly urge anyone considering a multishot solution (new or used, current or old, fuji hassy or sinar) to test it before pulling the trigger. It isn't coincidence that the vast majority of US museums, libraries, and archives have left multishot platforms for single-shot platforms over the last decade (before which they were a very significant presence). And when you make that comparison please make sure you're not just comparing the multishot mode of a camera to it's own single-shot mode; many of the multishot cameras came with raw processors with very mediocre single-shot raw processing algorithms. That, at least, will not be a problem for the GFX which ships with the very capable algorithms in Capture One for single shot. Though, I suspect you'll need a separate app to do multishot with the GFX, but I don't claim inside knowledge on that.
*It would, in theory, improve the pixel level color accuracy of subject matter that is 1) totally random 2) exactly one pixel in size and 3) perfectly sharply rendered and 4) falls exactly on the pixel grid, not between. That is not a scenario you'll find in real-world photography.
Doug, all good points.
Note, that FA on Phase one can't tolerate any movement either, and needs a totally static camera solution.
As for the Fuji and Pixel shift, your points are all well taken, and I agree that the key will be how it's implemented. If Fuji works out the solution in camera, and saves are a raw as the Olympus cameras do, I feel it could work very well, where as if Fuji takes the Son approach where you take the four shots and then have to open them in a Sony piece of software then save as a dog, I don't feel it will work as well. Also I would hope since currently Phase One/Capture One claim to have full support for the GFX camera, that when Fuji implements it, if it's a solution where the image is saved in camera, that C1 will eventually support it. Capture One has an excellent raw conversion for the existing GFX files, so I have faith that Capture One will work well on the pixel shift images.
As to your points on limitations, as a landscape shooter, I can think of many locations where wind would not be an issues (Monument Valley, Grand Canyon, Bryce Canyon, Any shot of a Colorado 14 foot peak) all come to mind. In my location, wind would be a factor for sure but I would still want to try it as I use FA in a similar work flow. FA can't tolerate movement, but there are many times I can combine parts of a shot in post.
Pixel shift on the K1 did not increase resolution, but did dramatically increase details, dynamic range and overall image quality. The problem with the K1 pixel shift was terrible raw support from both Adobe and Capture One. Capture One never supported it (what a shame) and the Adobe one and done was terrible. Where as many other software companies, with a lot less money and size were able to come up with excellent solutions that did allow for movement (iridient, Sliky Pix and Raw Therapee all do a great job).
I would hope that with the upcoming Fuji solution, the overall image quality will be improved, and dynamic range possibly improved as with Pentax. I have no issues with the need for a static camera as I tend to use a tripod most of the time.
Overall, for a 10k camera, if Fuji comes out with this at no additional change, it's still a great return on investment for any photographer using the camera. And in time Fuji hopefully might look at FA also as it's been proven by Phase One to make a considerable improvement to the overall image quality.
Paul C