There are many reasons why I would consider a camera with digital back over a GFX. Just as background, I use a CCD back on my Hasselblad V. It was a way to “extend” my old film camera. I sometimes shoot scientific images for work using that setup.
There are some things that I’ve learned by using a CCD back that aren’t always discussed since they are rather niche compared to DSLRs and even the GFX series.
The PhaseOne files from studio (tethered) require very little processing. I would say it’s an incredibly easy workflow/experience that maximizes keepers and saves time. The emphasis is on getting the output, settings and lighting right. The tonal transitions are beautiful and rich. I haven’t tested a GFX in the studio so I can’t say if my back is richer in color but I like the PhaseOne images which incidentally need minimal post-processing.
I like leaf shutters. I use strobes and the high sync speed is useful. I understand that you can use Hasselblad HC leaf shutter lenses on the GFX so if I were to use a GFX I would go with that approach.
I am used to film. I love the 6x7 field of view but 645 is not bad. Many medium format backs get closer to the 645 film size than the GFX. The field of view (if you shoot normally and compose to fill the frame) looks different with a physically larger sensor.
As some have already mentioned in this thread, these CCD backs have distinctive color profiles. While any digital file can ostensibly be retouched, the default profiles have a look reminiscent of the high-end fashion magazines from the last 20 years.
These old backs have plenty of negatives that I overlook based on how and what I photograph. They need a lot of light. I believe PhaseOne made the design decision to have rich colors and beautiful tonal range at base ISO instead of trying to squeeze out less noise at high ISO. In contrast, consumer cameras are designed for high ISO because consumers don’t carry strobes and camera influencers/vloggers judge cameras by the lack of noise at high ISO. Very few review sites judge a camera by the tonal transitions at base ISO. However, studio pros (fashion, food, editorial etc.) know how to light their image. It’s the lighting and composition that will set an image apart and the camera system that makes the image look the best is the winner in my book.
Another datapoint regarding the low ISO, I sometimes shoot with my digital back in natural light in the field. The low ISO (I don’t shoot above 100) limits what images I get. Unless you have bright sun, forget about fast moving objects. On the older digital backs the review screen is low resolution and downright hard to see, especially outdoors. The auto white balance is primitive. I think these are designed for the studio where one would tether and perform a white balance, get the settings and view the image on a 27 inch monitor when preparing for a photo session. But for field use, the poor auto white balance, low ISO and poor review screen are a challenge.
To conclude, if you shoot with a lot of light, those medium format backs may be worthwhile. If you shoot in the field with more variable conditions, the GFX may be a better choice.