To be honest, I find this somewhat amusing: for years there were complaints in many places that we're way too critical... just because we use the full spectrum of the star rating, so an average lens really just gets an average (3-star) rating. Which also means, we keep the very high ratings, especially the 5-star one, to lenses we think deserve it. This rating hasn't been given too often, yet. Now that we do, for the 2nd time, if I remember correctly, we're too generous
As to the Sigma vs. Nikon debate: the difference in rating is mainly because of the MTF performance wide open, this is where the Sigma lost that half star. The bokeh fringing, as explained already, is not tested in a very formal way, in real life (= in the field) the difference between the two lenses is insignificant, IMO.
And about that single figure star rating: honestly, I don't like it. And to my knowledge, Klaus doesn't either. It wasn't there when PZ tests started more than 10 years ago, there was just the written verdict at the end of each review, but PZ received so many requests to provide a simple/numeric final rating that we just couldn't ignore it. It turned out to be as misleading as we thought, because even though we provide fairly short reviews, that's obviously still way too much for some to read.
The D3x: I know it's dated. Trust me, I know very well, because it's still my main camera, so I (have to) use it quite often
The issue is, that we hesitate to change a test camera, because each time we do, we lose comparability with already existing reviews. That's why we didn't upgrade to the D800E (or the D810 later) when it was announced, because the fairly moderate increase of resolution per axis just didn't justify to more or less start the FX reviews from scratch. If we had known back then that 36 MP would be the high-end mark for Nikon for that long, we probably should have decided otherwise, yes. However, NOW it makes absolutely no sense to switch to a D810, because it is very likely that something with higher resolution is on the horizon. D810-based reviews would just be a waste of resources, because we could only do a few of them until we switch to whatever is hopefully announced later this year.
I know it's sometimes hard to be patient... I'm actually suffering with you. The plan was to switch to a high-MP Nikon last year already... but unfortunately Nikon did not adjust their roadmap to our plan