The Fuji GFX 50S should by definition have better IQ than the A7R. Don't get too carried away. The problem with the XT2 is if you don't like the XTrans sensor in post-processing (I don't) and that it's a smaller sensor than the A7R II. I'm not sure how the A7R mk I compares with the XT2 though.
Well the GFX with it's huge lenses is bigger than a DSLR outfit...no way I'd take it traveling.
The A7R delivers much better and more malleable images compared to the XT2. I tried out the XT2 for a week...did not like the images I got out of it.
In general, APS does not match up with a full frame sensor when you make large prints. I tried and the APS just runs out if steam. They are fine if you print small or just display onto a screen which the majority of APS shooters do.
I do a lot of serious landscape work and truthfully I have never met a single photographer shooting a Fuji system...lots of Nikon 810 and Canon 5dsr and some Sony cameras, but no Fuji or Olympus cameras.