I didn't have the GFX100 and X1D at the same time, but both cameras (and the X2D) overlapped with my Leica S. While I believe that any of these cameras could produce indistinguishable images, sometimes it feels like you "can't get theah from heah". You're just not starting in the same place in RAW converter space (whatever that is). It's not too hard to get the Hassy and Leica images to feel similar (with a little violence done to the reds, default sharpening, and contrast), but the Fuji images were from a different planet. Not bad, certainly. Sometimes striking. Often beautiful. I think some of it is the lens design. I say that only because I felt much more comfortable with images taken with the 250/4. I usually shoot the very wide end, and the GF 23mm left me completely cold.
About Phase One: I made a very serious error. I tried - and loved - the IQ160. When I finally decided to buy a tech camera, back, and lenses, my budget fell short. It was P65+ or IQ140, and I went with the smaller sensor. Not a success. First, I missed the acreage of the bigger sensor, and then the value of the IQ140 plummeted as soon as the Sonys got to 40MP. The last straw was when Phase offered a special upgrade program for owners of the P65+, but not for the IQ140. So I dumped it all JUST before live view became available. Yes, the Leica S had AF and f/2.5 lenses, and that was my justification, but If the IQ3100 had been around, I think I would have stayed with Phase One. Of course, once you fall off that wagon, it's stupidly expensive to get back on, so I'm left with "only" the Leica S3 and Hassy X2D. (I know, tragic, right?)