I have been shooting with an A9 since it came out, it's been my favorite camera. I would say the Z8 is on par with the A9's performance as far as speed and AF. I really love the adjustable zone AF system Nikon has, it suits my shooting style much better than the big box Sony employs.
The camera is much faster in use, from selecting AF points, to turning on, to formatting cards. As others have said, the buttons on the Sony are much more customizeable, but the Z8 is rather easy to setup. I love the F buttons on the face of the camera, so fast to get into crop mode, or change iso. I find the EVF a bit low res compared to the Sony's but do like the bigger LCD.
The camera does a superb job of shooting video, it's better than my A7s3 in many ways. I LOVE the ability to crop in when shooting in 4K, something the A7s3 can't do. i got the 24-120 with my camera, it's razor sharp. I could shoot with that lens for everything, I find it better than the 24-105 OSS. My sony lenses also work great with the Megadap.
In conclusion, there is nothing in Sony's current lineup to match the Z8, despite reports of it's not so great AF. At some point, I can see myself selling off the rest of my Sony gear, and picking up another Z8 or Z9, with a couple more Z mount zooms, and keep that kit for the rest of my career.
