Just saw your comments, and I have a few thoughts on this.
First, the image sensors for the Fujifilm, Hasselblad and Phase One backs, are all made by Sony. Even the Nikon Z series sensors and presumably, the Leica SL2 and M11 sensors are also made by Sony. Inherently, they all have the potential to deliver the same IQ, for the same generation of sensor technology.
So the IQ differences come down to the optics (lens design and manufacture), camera design and manufacture, and firmware, all specific to each camera line. Between Fuji and Hasselblad, for instance, it’s clear what choices they made: Hassy went for IQ, and Fuji went after market share. These decisions have consequences.
As for Phase One, obviously, there is no such thing as “sharpness of the Phase One IQ 150”. You can only talk about the IQ4 150 in the context of which camera and lenses it’s used with: with the XF + compatible lenses, or a technical camera + compatible lenses.
With the XF, many of the Schneider lenses are indeed old designs, built originally for 50-80 MP backs, and are getting long in the tooth. IMO, the lenses that are real standouts today on an IQ4 150 are (in no particular order) the Schneider 80mm f/2.8 (Gen-2), 120 f/4 Macro, 150 f/2.8, 45mm f/3.5 (flat field), and the 120 f/5.6 APO Digitar Tilt-Shift, which is actually a very old lens, but surprisingly good even on a 150 MP sensor. There is also a Schneider 240 f/4.5 lens that probably very good – I have never tested it. The Schneider 35mm f/3.5 (curved field by design) is a half-notch lower, but still very good. The only Fujifilm lens that measures up to these lenses is the 250mm f/4. The rest of the Schneider primes are definitely a notch lower.
In any case, the DSLR is dead as a camera paradigm, and these bulky and heavy Schneider lenses with “huff-and-puff” auto focusing just don’t make sense. A lot of people use them in studios (or on location) on tripods, using tethered live view, typically for glamor or fashion photography and portraits that eventually make it to bill boards. Corner to corner sharpness is irrelevant for these use cases. Even the Schneider 110/2.8 and 150/3.5 work very well for these kinds of uses, although both lenses suffer from LACA that would be unacceptable for landscape photography.
It is an entirely different story with the tech cameras. For a niche market, there are actually six(!) of them: Phase One, Alpa, Arca Swiss, Cambo, Linhof and Silvestri, all of which work with Phase One backs. (Hasselblad also used to be in the mix before their litigation with Phase One, so now they have their own digital back cameras, although I don’t think any of the Hasselblad cameras is a tech camera, per se.) All these cameras have different degrees of Shift, Rise / Fall, Tilt and Swing capabilities, either built into the cameras or customized versions of lenses (chiefly Rodenstock). All except the XT get a little geeky, and some of these tech camera setups look like they were yanked out of the wall of a Borg ship from Star Trek.
The XT has made it very elegant and simple to use a tech camera, but limits Shift / Rise-Fall to ±12°, and there is no Tilt / Swing. I recently had a chance to see and play with it, and from what I’ve seen so far, it is very easy to use. The lens selection for the XT now comprises most of the Rodenstock portfolio – 23, 32, 40 (with Tilt), 50, 70, and 90. The Cambo version of the Rodenstock 135mm is also usable on the XT, but with a cable to use the X-shutter. There is also a rumored 150mm in the works for the XT.
People who have used these lenses and made large prints swear that the Rodenstock lenses with an IQ4 back is the very best there is, and nothing else compares with this system. I saw a bunch of large prints of images shot with Rodenstock lenses and various tech cameras, and they do look very impressive. However, I don’t have any basis to compare those to how prints of this size might have looked if they came from a GFX 100S, for instance.
Besides the IQ4 150MP color, there is also an IQ4 150MP Achromatic, which is not just black and white – it also includes IR. This back has to be used with a filter (Visible Pass / IR cut off, Visible + some IR, Visible cut off / IR pass only). There is no other sensor like this made by anyone else – I’m told that this sensor was designed to catch some incredibly small (single-pixel) differences from frame to frame for some scientific application. I saw a 6-foot wide print on the wall at a dealer’s shop, and it was just jaw-droppingly amazing to see a print that large with so much fine detail.
The question is, how many people have a need to make 6-foot wide prints, and how sharp and fine-detailed image files really have to be. At the end of the day, the single biggest part of photography is the consumption of an image. If the image is primarily for online publishing or for making smaller prints, there is very little a Sony A7 IV or Leica SL2S couldn’t deliver with a handful of lenses. Sony cameras have some really world-class lenses, like the 12-24 f/2.8, Voigtlander 35/2 and 50/2 APO Lanthar, 100 f/2.8 (T/5.6) STF, and 135 f/1.8 GM. If you aren’t making prints larger than say, 16” x 24”, I don’t know if you need anything more than a Sony A1 or A7R IV (or V soon). Or a Leica SL-2 and the f/2.0 SL primes (28, 35, 50, 75 and 90 so far, 21 and 24 coming). But you’re paying a huge Leica premium for these.
But whether you print your images or just want to capture them for posterity, if you aspire to capture images that even begin to approach the incredibly subtle tonal variations you see in the photos in Claude Fiddler’s book, my gut says your best shot would be with a Phase One XT + IQ4 150 + a couple of Rodenstock primes.
My 2 cents’ worth…