There is one incompatibility that is bothersome, the SB900 AF Assist light does not trigger when mounted on a Z body. The only AF Assist light available is the small green light on the body that is blocked by any of the better lenses. For those shooting events or other flash uses, AF is depended on ambient light and not the very effective patterned near IR light put out by the SB series of flash or any of the brands that are compatible with the SB control logic.
I have 7 flash units, 3 SB900 and Yongnuo YN-968n and another I forgot the brand name. All of them work well with my wireless triggers and commander from Yongnuo 622 Tx master and tranceivers. They all trigger their assist lights in the shoe of any Nikon DSLR but none of the Z camera send the serial signal through the shoe contacts to trigger the light. I asked a forum about this and was told no mirrorless trigger assist lights because of some filter in front of the sensor.
That made no sense so I set up a test in a completely blacked out room, and mounted a D850 on a tripod and used my Z6 hand held to attempt to focus on a target in the pitch black room with a focus chart on the wall. I wanted a baseline verification that the Z6 would not focus at all Then I mounted a SB900 on the D850 and pressed its AF-on button to initiate IR assist light. I put the Z6 in Focus priority for shutter release, pressed the release and as soon as I pressed the AF-on button on the D850 the Z6 snapped into focus and triggered the shutter. So the sensor has no IR filter and the rest of the AF system responded to the patterned assist light.
I connected an oscilloscope to the shoe contact to read the serial data stream of the D850 by connecting a remote flash cable with shoe male and female contacts at the ends and after lots of test conditions to isolate the data pattern to flash assist light trigger I tried capturing the same stream from the Z6 and it was not present. Later someone posted on a forum that one model of GoDox speed light with a new firmware update worked but I never saw any replies by the OP when a number people wrote posts asking for more details.
That was 2 years ago and I was not happy because I shoot a lot of events where ambient light is poor so depend on that IR assist light for accurate AF. I never tried it in an event so took both the D850 and Z6 so I would be sure that at least one of the cameras would work. As it turned out, much to my surprise, the Z6 could AF on much lower light than the D850 and shot the whole event with the Z6.
I wrote to Nikon asking about this problem and they responded with a form letter saying they would pass on my comments to Japan. I figured they changed the data stream to only work with later Z dedicated flash so people would have to buy the more expensive Z enabled flash instead of the very competent 1/6th price Chinese flash. I like the SB900 but here they are $700 each and my $120 YN968n has built in video light and 2.4ghz 622 transmitter master/slave receiver built in.
The bottom line is that the Z6 can AF down to -6ev in AF-S mode so I can use it in pretty dim light but I still want the patterned assist light. In the studio it would be nice to not have to use the modeling lights all the time so I still want an assist light that works with Z bodies because the D850 and 18 top lenses are going to be sold to cover a Z7 2 plus 70-200 2,8 S and a 50 1.2 S so it will be an all-Z kit and all top S lenses.