Just out of interest have there been any reports of poor AF with any Nikon mirrorless cameras when using long lenses?
Of course there are. There are reports of poor AF on every latest camera model ever made, including the top-of-the-line Canon, Sony, and of course Nikon models. No AF system is perfect, and a lot of times some people simply do not understand how to use those cameras.
For example a burst of pics of a bird in a tree when the focus changes very slightly during the sequence from in focus to just soft oof?
I am not currently a Nikon user but just wonder if the AF is really good or not.
I have the Z6, Z6ii, Z8 and Z9. In my experience, AF on the Z9 and Z8 is really good. AF on the Z6 is fine for static subjects, but I wouldn't use the Z6 to track most moving subjects, especially birds in flight. Before the Z9 was available, I was sticking with my Nikon DSLRs.
For example, the king vulture flight sequence I posted to a recent image thread is 4 selected images from a sequence of about 70 images. I captured that sequence with a Z9 and a 400mm/f4.5 Nikkor lens at the top 20 fps. It was a rainy day (you can see water droplets around the vulture) and the sky was dark, but the Z9 did a great job. Every image among the 70 is sharp over 3.5 seconds:
https://www.dpreview.com/forums/post/67445257
I have also done 120 fps JPEG in that same setting and the result was great also. However, I have had occasions that the Z9 lock into the background.