I shot the Z6 I and R6 in the same situation, side by side. The Nikon used the Z 85/1.8, and the Canon used the RF 85/1.2. Light was moderately low, but not too dark maybe 1/200, f/1.8, ISO 800 or thereabouts. I used BBAF on both cameras. Subjects were dancers on a covered outdoor stage in sunset light. The R6 was better in some ways but about the same in others in terms of AF.
When talking about AF performance, I separate it into two areas: acquisition of the subject or placing and keeping the AF point on the subject, and tracking once the subject has been acquired.
In terms of acquisition, there is no doubt the R6 was better: if you turn on their optional feature to start tracking AF on the AF point, you can place the point on a subject, and the camera will keep the focus point on the subject following it around the frame for as long as you keep the AF button pressed. It was pretty sticky, but could become unstuck too. This was great for picking a subject and following it around a busy scene. If it messed up and let go or jumped to something else, it was easy to fix by letting go of the AF button, placing the point, and pressing the AF button again. Until the Z9, Nikon had nothing like this in the Z line. With the Z9, 3D AF works like this, but perhaps better. I haven't used the R6 since that shootout to be able to compare it to the Z9.
In terms of tracking when you keep the AF box on the subject, both cameras were surprisingly the same. The 85/1.2 vs. 85/1.8 could be a confounding factor here, but what I found is that the Z6 actually had a slightly higher hit rate than the R6 when the subject was approaching me at moderate speed. This wasn't about small misses (eg. eyelash vs. pupil), but really obvious misses. This was surprising to me, but I'll take it. This also means I could make the Z6 work as well or better than the R6 if I worked harder at placing the AF box correctly.
In the end, it wasn't enough to make me switch, but the AF was only part of the reason. The Z lens system for what I like to use was built out at that point already and of really high quality, with the f/1.8 prime lens line there. Canon then barely had any primes in the same range, and what it had was either huge and expensive (f/1.2 primes) or mediocre (the two f/1.8 semi-macro prime lenses), so switching was expensive and would have been an overall downgrade for me. The bodies were also more expensive.
I use a Z9 now, and the AF is pretty much all I can ask for. When I first got it, I started using 3D AF since that's what we were all waiting for, but over time I found I liked the W and S area AF modes instead, especially with face- and eye-recognition. The camera recognizes human bodies, so even if I place the AF box on a person's torso, it will put the eye AF box on the person's eye which is outside the main AF box. Basically, I just have to identify a human body to the camera, and it will automatically find the eye, and this has made it much easier to acquire focus especially when things are changing and moving quickly, like photographing a fast dance scene.
One big thing the Z9 fixes is shooting in very dim situations you might have when shooting with flash in a studio, where the ambient light is very purposely underexposed. The Z6 I and II could not focus to save their lives in this situation, but the Z9 is effortless, especially if the subject is already in the frame. If the subject was off-frame and quickly came in (eg. a dancer getting a running start on a jump), then it might miss every so often perhaps due to additional AF acquisition time.