The tracking/auto focus systems for the D850 and, in my case, D750 are rather good when taking into account the period they where developed.
I have/had very good results using, especially, the 9-point dynamic AF (left<–>right movement) setting on my D750 and also the 3D tracking (front<–>back movement) setting. The D750 lacks (human) face tracking support.
The downside, for as far as you can call it that, of these systems is that they target a somewhat wider area and not a spot. Targeting a bird in motion using the dynamic 9 points method, for example, might focus anywhere inside the 3x3 grid (even the background if you’re unlucky). There’s also no full tracking involved: the targeting is done around the fixed/static focus point set by the user. It is up to the user to keep the object within that 3x3 target area.
The new system that is incorporated into the Z series is able to, exceptions not withstanding, spot focus on eyes, wherever the eye is found within the full frame. It is really good when it comes to human eyes. But with animal eyes it, obviously, varies: There are many different eye types/shapes and the algorithms seem to be focussed on cat and dog eyes. YMMV if you shoot other animals. It does have a nice full set of alternative ways to track and find focus, though. Combine that with human skill (panning/tracking by hand) and you’re able to take great shots.
The new auto focus system is a really nice step forwards and makes shooting a bit easier, but, depending on the situation, you shouldn’t expect miracles. The image you linked to is a lucky shot insofar that I consciously, I was testing my Z6ii’s tracking AF, relied on the auto tracking and focus to do the work for me. Under normal circumstances and in this specific case I would have used pinpoint AF and relied on my own skills.
