Compared to a D850, here are things where the Z7 is arguably better:
- Smaller and lighter (though once you start putting long and heavy telephoto lenses on, the body advantage isn't so important)
- The first S lenses are indeed superior optically to equivalent F-mount lenses (e.g. 35mm f/1.8 S versus 35mm f/1.8G)
- WSIWYG finder (great for black and white shooting)
- Silent/No slap mode (and overall the camera is a bit quieter in mechanical modes, too)
- Live histogram (though not at same time as some other things, like virtual horizon)
- Focus peaking in viewfinder (but only in MF or in manual override of autofocus)
- Easier to focus manual focus lenses, by far, due to peaking, rangefinder, and magnification abilities
- New Diffraction Compensation control (not fully tested by me yet; working on it)
- On sensor VR works (but buried in menu if no lens switch)
- New focus peaking shot review for Focus Shift Photography
- Timecode for video available
- 10-bit 4K video with N-Log available on HDMI slot (though you'll overheat camera at some point)
- Better video autofocus, by far
- Ability to offload images via Wi-Fi in both AdHoc and Infrastructure modes (still testing)
- New i-menu that's customizable (D850's is fixed and different format)
- (Arguable) Use of U1, U2, and U3 to configure camera quickly
- (Arguable) Strange and as yet unexplained application of different defaults in Adobe conversion programs
And some disadvantages, where the D850 would be arguably better:
- No ability to override automatic DX crop mode (some DX lenses can cover full FX frame)
- Smaller buffer
- No second card slot (for Raw+JPEG, backup, or overflow)
- Buried Metering Mode, Picture Control, Qual in the menus
- Likewise, Focus control buried in menus (or needs to be promoted to a customizable control, using up one of the fewer ones available)
- No two-button reset or format
- Nikon does not recommend user sensor cleaning
- Worse battery performance
- Poor ability to control continuous autofocus options, particularly tracking
- Slightly fewer Retouch options