I thought I would mention my own experience with the Fuji XT-3.
As I have gotten older, the idea of a much lighter camera is extremely appealing. I purchased a Fuji XT-3 and had significant issues over the six months I possessed this camera.
First of all let me state that I’m an avian photographer that I augmented my Nikon gear with the Fuji camera. Most recently I had been using a D 500 with a 200 to 500 mm telephoto mostly handheld. This camera and lens weighs almost 3 pounds more than the Fuji XT-3 with 100 to 400 telephoto. And ideally, this made a huge difference in my ability to follow birds in flight.
I purchased the camera in October 2018, as the dark months of winter the Pacific Northwest were setting in. What I found was the Fuji XT-3 with the 100-400 telephoto had significant issues with focusing either birds on a stick or birds in flight.
It simply would not focus effectively in lowlight conditions from its minimal focal point to about 25 feet. It was slightly better beyond 25 feet but very slow. I was never able to get a bird in flight in focus.
I sent it back to the factory in New Jersey four times over the six months I had the camera. The factory changed the firmware settings. This made little difference. They were unable to find anything wrong with the camera but I never sent the lens and camera combination into the factory.
Another person at the store where I purchased the Fuji XT-3 was having similar problems with their camera.
I experimented with multiple menu settings and was never able to satisfactorily find one that worked in the lowlight conditions that I experience the Pacific Northwest.
Out of frustration I was able to return the camera and trade it in for a Sony A7R3 which has been much more satisfactory although lowlight conditions also affect this camera with focusing.
Was it me? Due to my unfamiliarity with this camera? Is it the basic premise behind mirrorless cameras in lowlight conditions?
I’m not sure but the Sony does seem to work better than the Fuji and the Nikon focus is better than both of them in all tests that I attempted.