Dpreview tests the Fuji GFX and says the advantage over the A7rII is negligible…

DPreview  added the Fuji GFX studio samples. The camera uses the Sony 51MP medium format sensor. But despite the size advantage there doesn’t seem to be a big quantum leap over the Sony A7rII:

“In terms of high ISO noise, the GFX 50S performs fairly similarly to the Pentax 645Z and, as sensor size alone would lead you to expect: better than the Canon EOS 5D SR. However, because Sony’s a7R II sensor uses a more modern BSI design, it’s able to be more efficient, which means it’s able to close the gap to the bigger sensor cameras. “

As I stated many times I think Sony should use the much larger 55x41mm sensor if they ever are going to make a medium format RX or system camera. That should really give us a visible difference in both, quality and esthetic of the images.