First, understand where I'm coming from - 99% of my work is on tripod, 99% of my subjects only move in the wind, not by themselves, and image quality is important to me and sometimes I'm required to make big prints. My standard for quality printing is 300 pixels per inch which even with 51.4 MP is only28 inches on the long side, not that huge a print. Of course larger prints get viewed from further away so really big prints look just fine thank you.
All cameras have flaws - that's what makes deciding on what to buy so difficult, and of course, if we wait long enough, something better will come along, often just after you purchase the camera of your dreams.
The Pentax 645 has been my working camera for the last three years and it has worked very well, mostly. Shutter shake limited the use of long lenses, sp. the 200 and 300.
Fuji electronic first curtain fixes this entirely.
I used it in live view almost all the time, and the lcd does tilt upwards, but not when the camera is vertical.
Between the rotating viewfinder (unique to the GFX, and the tilt and swinging LCD, the Fuji is fantastic.
IN using live view, I need to magnify - that's easy, but I also often need to do a focus blend, and that often involves the nearest part of the subject at one of the image edges. Getting the magnified spot to the corner is painfully slow, and once it gets moving, it likes to continue drifting past where you wanted to stop if it isn't exactly on the edge.
the Fuji GFX is much faster to move focus spot, uses either the joy stick (great) or double touch to magnify - awesome. This makes taking every photograph easier.
I can't comment on autofocus as I have no native lenses yet - not even sure I'll rush into getting any. The Pentax lenses are remarkably good, and even the 35 that I had some concerns about is better than most good zooms.
The Pentax sometimes lied about exposure, the histogram not reflecting reality - a but in the firmware I suspect, no such issue with the Fuji and swiping to bring up the histogram is so nice so my exposures are noticeably more accurate.
Of course, reviewing images on a camera with a good touch screen is great, and checking focus by quickly enlarging and moving round the image very helpful.
For the work I do, the camera is great, with no significant flaws. That might not be your mileage.
I wouldn't try to photograph the dog at the off-leash park with it - and for the amount I'd save on not buying a full set of Fuji lenses, I could easily afford almost any other sports oriented camera for those tasks.
Battery life is excellent. So far, I've never even got close to losing power though eventually I'll get a back up battery to be safe.
What are the problems with using the Pentax lenses - well, I don't miss auto focus - as I always used them in manual mode on the Pentax anyway. I do miss the camera not recording the focal length of the lens as I need that information when stitching. Fortunately I usually remember, and am looking at the voice recording facility of the camera to jog my memory.
People make a lot of the shallow depth of field of larger than full frame formats. That's true, but the difference in magnification between full frame 35 and the Fuji GFX isn't that much and so far hasn't been an issue - I focus blend where I did before.