There has been a bit of "luck of the draw" about how a particular copy of any lens performed.
I had some interesting adventures and good and bad luck with long variable focal length lenses.
My Nikon 200-500 went back to Nikon 3 times under warranty and was much better when it got back the third time. At the same time I had a Sigma 150-600 Sport and my copy was optically equal to my repaired copy of the 200-500 and had that extra reach but was heavier.
Then the Tamron 150-600 G2 came out and I tried the first 2 copies our now closed local camera store got in. One copy was very noticeably better in both IQ and focus speed and I bought it.
After using all 3 for a while I preferred the Tamron 150-600 G2 to both the Sigma and the Nikon. I sold the Sigma to a member of my camera club who had one on order for several months and he wanted it to shoot an air show that weekend, he made me a great offer that I gladly accepted.
I kept the Nikon 200-500 until I replaced it with a Sigma 60-600 Sport that was the personal lens of a camera store owner with a bad back who kept his Tamron 150-600 G2 and newly arrived Nikon 500 PF and sold me the heavier Sigma at a price I could not refuse. I ordered a Nikon 500pf from the dealer and he sold my 200-500 to one of his customers and waived the normal consignment fee. I used the Sigma 60-600 on my D850 and the Tamron on my D500 during the long wait for the 500pf I had ordered from him to arrive. After using it for a month Sigma had a mother board fail and Sigma replaced the lens.
When the 500pf arrived I got a great offer from a camera club member who wanted the Sigma 60-600 sport to use at an air show he was going to with the guy who bought my Sigma 150-600 Sport ... I actually sold the 60-600 for a little more than I paid for the one that Sigma replaced.
I had a friend who tried my Tamron 150-600 G2 and loved it so she bought one. Her copy was hit and miss on AF focus speed and IQ reminded me of the one I did not buy. She sent it to Tamron and they replaced the lens but in the meantime she bought a Nikon 200-500 that was a superb copy. When the new Tamron 150-600 G2 arrived she gave it to her son who loves it. Her husband bought a Tamron 150-600 G2 at the same time his wife did from the same dealer and his copy was a great one.
My Tamron 150-600 G2 was in the last batch of f mount lenses that I sold when I went to all Z mount lenses.
WOW Ken you have been around LOL, my friend she has the Tamron 150-600 G2 and uses it on the D850 and D4s, mostly for birding and she is blown away with it, i hear lots of good reports about it from other club members as well.
I think its what ever makes you happy and works is all that matters, there isn't a lot in it overall.
When buying my 70-200 FL i found that the Tamron had better VR than the Nikon the Nikon had more accurate or consistent focus.
If ever your curious to find out whats inside some lenses as well as build quality chat with and watch a friendly independent technician, of course buy them some lunch etc, its surprising what some of the lenses have inside. I spent a little time also watching the calibration process of a variety of prime Leica and Ziess lenses, its amazing some of the tolerance levels set compared to other brands. i had the technician run the gear over some of my lenses, about $50 each.
Mostly primes, the 200-500 was much more stable to adjust than the 28-300.
Yes defiantly lens sample variation can be a real frustration at times, hence my hobby horse of really serving it up to manufacturers, they take your money you should respectfully give you what you pay for.
I believe pressure on manufacturing volume output sometimes is the cause of the variation in samples, time is money, in cases depending on the product its more profitable to punch out volume and replace faulty units back from the filed, i call this marginally outsourcing QC or cost cutting in exchange for increased volume output, i mean this doesn't apply to everything made or exotic items and i am
NOT saying this is the case with Nikon.
Z glass when on a Z camera has clear attributes, we have always known that, the 200-600 should in this case have a advantage over the 200-500 how much, we will have to wait and see.
The 70-200 F2.8 Fl versus the Z version seem so close to one another in performance its not worth investing into the Z version, this has been the coalface feed back by some shooters, while unique and telling its not the case with every lens.
I would like the 200-600 to be like the 100-400 or at least the size and weight of the 500 pf.
Only an opinion