My primary wildlife camera is the Z9. The Reasons include:
silent-shutter;
high frame rates;
PreCapture, image quality (including cropping flexibility);
full frame AF coverage;
Subject Detection, especially eye-detection;
Autofocus works at fStops higher than f5.6, even f9, f11;
No more AFFT;
Last but not least, we have game changing options in the Z Nikkors for wildlife photography. These are the choices in lighter options for long focal lengths (600 PF, 800 PF especially) as well as Ultrawide zooms.
However I use my D6 primarily for the advantages of its cross-type sensor based Autofocus plus the less widely know about AF features. Collectively, these empower the D6 with basically instant lock on, and extremely robust Close Focus Priority. Remarkably as it may sound, the subject Detection on mammals and birds is outstanding in the D6 (embodied in how pattern recognition works in the eye and face detection settings). These features further distinguish the D6 as unique among all DSLRs. One sees these advantages especially on fast moving/erratic subjects within vegetation.
These differences in AF are well established, although MILC Autofocus systems will probably close this gap in the next generation. One can achieve high success rates with a Z9 in these situations, a Handoff/AF Override setup closes this gap - Custom Area modes with Recall Focus Position
I detailed the differences wrt the D6 Autofocus, including cited technical references, in my posts in this thread
I'm not sure how many people actually subject their systems to test it? When I look at the image at normal magnification, it appears in focus. Now pixel peep at 100% and it's all too apparent that the observable focus point was different than the one indicated by the EVF green square and the...
bcgforums.com
Similarly to the D5, the D6 sensor has inherently better image quality at high ISOs, particularly in shadow recovery, but obviously less flexibility for cropping. Here lies the real world advantage for lowlight conditions, at least without the workloads of post processing Z9 images for noise reduction etc.
Actually, in Lowlight conditions the EVF of Nikon Mirrorless cameras has big advantages over even the brightest OVF of a DSLR. The differences are obvious with wysiwyg Settings; and with correct exposure the Z9 Autofocus works well in my experiences.