Sometimes I look into George Barr's Blog "Behind the lens". He's an owner of the Pentax 645Z and reported about his experiences in the past.
Now I discovered that he got a Fuji GFX 50s. He uses it with his Pentax MF lenses via Pentax 645 to Fuji G adapter from Fotodiox and wrote posts about it.
Since I know that there are theoretical discussions within threads concerning the Fuji GFX compared to Pentax cameras I thought there may be PF members interested in Barr's comments. You can get to his first blog article about this
here.
Regards, acoufap