I've been using the 500mm f/5.6e on the d850 and Z7 for a couple weeks now. Here's my opinion on it.
It's very light and easy to carry and hand hold. You all knew that.
I've gotten some great photos with it. For example
this heron (d850) and
this owl (z7, 1.4xtc, 1/40th sec, tripod).
On the Z7 for those owl shots, the autofocus was slow, but it did seem to latch on the eye very well even with foreground clutter. It worked very well with the TC 1.4x on the Z7 on a tripod with some manual focus adjustments now and then. The live-view magnified focusing really is a winner with this combo, and being able to shoot in silent mode lets me use really slow shutter speeds even at 700mm (with IBIS + VR on).
My main issue with the lens is that the focus is not too fast. It's OK on the d850, but it can have problems on the Z7. Hopefully the next firmware release will help. I'm holding my breath until then.
I did a few non-scientific tests with the d850 and z7 and the Nikon 70-200 f/2.8e, Tammy 150-600 G2 (at 600), and 500 f/5.6e. I switched between two focus points. One was in normal interior lighting and was a white box with black letters. The other was an unpainted wood fence in overcast daylight.
Results (using AF-C and single point autofocus):
70-200 was near instant on both d850 and z7 for both targets.
The Tammy on the d850 on the outdoor target is a little slow, but does not seek. On the indoor target, it might start going the wrong way, then correct quickly, then lock on. A bit worse than the outdoor target. On the Z7, for both the indoor and outdoor targets, it usually does one partial seek the wrong way, then corrects and locks on, though slower than the d850. Also, once it locks on it still wanders in/out a little in AF-C. On both targets, the Z7 is not very consistent, sometimes it's worse than I described.
The 500mm f/5.6e on the d850 is pretty fast on the outdoor target, faster than the Tammy. It does not seek, but goes directly into focus. It is not instant like the 70-200, but it's good. On the indoor target, it usually partially seeks once then locks on. It's a bit faster than the Tammy and does not wander in AF-C. On the Z7 for the outdoor target, it's almost as fast as the d850. Maybe hunts a tiny bit to get fine focus, but locks on well. On the indoor target, it is a disaster. Hunts back and forth multiple times through a large range, and sometimes does not lock on for a long time. I'd say it's unusable.
Marc