Yeah I’ve made the move from Sony to Fuji in the past to release some money, still preferred the full-frame system so ended going back to Sony.
I’m looking a possible move to Nikon to pretty much do the same, scale down my kit cost wise and release some money back into my piggy bank.
The Sony system is great but so is pretty much the Fuji system for different reasons.
If I was you I'd wait til the gen 2 Nikon. The Z's are great, and AF is much better than I and many initially thought/anticipated, but it's not quite as good as Sony. I think by the Gen 2's they'll have an extremely competitive lineup of cameras. The Gen 1's are truly impressive for a first attempt so imagine what the gen 2's will be like
One area Nikon could compromise on is the release of its af protocols to third parties. The Sony system has some amazing 3rd party lenses and plenty more in the pipeline with none of the associated problems that typically exist/ed with dslr such as af accuracy/fine tuning.
I can see Sigma Arts being released for the Z system, but I'm not so sure about Zeiss etc. That being said there is a Sony to Nikon adapter so I'd imagine you can use Sony Zeiss lenses on Nikon.
Phase detect can have inaccuracies whatever the system.
You can still need AF fine-tune on mirrorless cameras if they use on-sensor PDAF for AF-C focusing, don't ask me why because I don't know, I would think if the PDAF was on the sensor it should be as accurate as CDAF.
The reason that PDAF has inaccuracies is because it uses complicated algorithms that take into consideration distance, movement etc etc and is not purely based on bringing two images together or relying on contrast. This makes PDAF faster, but with the possibility of tolerances. If you then combine this with a lens that is not calibrated to that specific camera it can exaggerate the tolerances.
Most mirrorless negate this as much as possible by combining PDAF with contrast detect (hybrid AF) so you get the speed of PDAF with the accuracy of contrast detect (in theory). When I spoke to Nikon they advised me that the Z series use hybrid AF for all applications except pinpoint AF which uses contrast detect only. This would suggest to me that contrast detect is still considered more accurate than hybrid, although Nikon would not divulge any more info on the exact mechanisms (understandably so). I then went on to ask them why they have AF fine tune if a form of contrast detect is always used and therefore should be accurate. Their response was:-
"
The AF-Fine Tune feature is not used to correct a problem, it never has. The idea is that AF-Fine Tune will allow a user to adjust the focus based on their own ideals which may be slightly different than what the camera is producing. If your lens has a focus problem you should return it to Nikon Service as AF Fine-Tune is not intended to solve optical problems which will generally be outside of scope for this tool"
Now I've always used AF fine tune to correct front/back focus, but that's because I couldn't be bothered to send all of o my gear back to Nikon every time I bought a new lens, but also realised that if you used AF fine tune it
could be a little less predictable. I know a Canon user that has some serious kit (probably worth more than my car) who does just that, send his gear off all the time, but then he has a 24 hour turnaround.