What are the significant usability differences you mentioned?
I hate to hijack the thread from the D850 and turn it into a Sony one, I was simply pointing out that it's not unusual to be impacted by differences that you got used to with a new camera when needing to go back to an old one but since you asked, here are a few that come to mind at the moment:
- Focus point joystick - you don't need to first push a button and then move the point with a click on a buttoned wheel - much better direct control
- Battery that you don't really need to worry about constantly
- MUCH better AF at the edges of the day (things that the a7R3 easily focuses on the a7R2 hunts on and then gives up)
- It actually tracks focus on moving subjects
- Programmable zebras that allow you to actually place the zebra threshold where the RAW file starts to clip - you can actually program it to a specific value or a specific range, not just 70, 80,90, 100, 100+
- In my case my a7R3 level is much more accurate than my a7R2 which is off by about 0.5 degrees
- My Menu lets me get to things I need quickly
- By just moving the rear screen out a touch you can prevent the bill of your hat constantly switching the camera to the EVF when doing things on the rear LCD
- Just lots of little things you don't read about in the reviews based on a couple of hours of use at a Sony paid event...
I have plenty of gripes with the a7R III too but it is a significant step from the a7R II. I have a very detailed review that talks about all of the problems as well as the good here:
http://www.ejphoto.com/Quack%20PDF/Sony%20a7R%20III.pdf