I would agree it's center sharp, but it's a shift lens, so I guess I looked for different aspects of the lens, as I have shifted for larger resolution since early 2003, (don't do it as much now as most shots I want can be stitched without needing shift due to vast improvements in software)
But I can't argue with Theo's testing, as I feel he is very through, however I also am through, however I was looking for a different output, i.e. excellent corners shifted, as there are plenty of great standard 24mm lenses, and I would have to say the Sigma Art 24mm would stack up very well to the Nikon (that if you get one that is not off center).
When I moved to Nikon, back in 2012 with the D800, I immediately sought a great shift lens, and tried the Nikon 24mm, as mentioned I tried 4 different copies, 2 new 2 used, and never found 1 lens that gave me the full sharpness on shifts I had with the 17 and 24mm Canon TS-E II lenses. Of course I was testing on a 5D MKII and 6D vs D800 so there was more stress on the Nikon lens due to resolution. Personally I did not see what Theo saw, but I can agree that there are more than likely good examples and bad examples of all lenses. I know of another great photographer on another forum, who swears by his 24 PC-E and I have seen examples from him, so it can be good. I just got tired of testing lenses I guess.
Based on Theo's notes, I need to re look at the Samyang also.
My results correlate much more with the lensrentals findings on the Nikon 24 PC-E.
http://www.lensrentals.com/rent/nikon/lenses/specialty/nikon-24mm-f3.5d-pc-eThis has gone a bit out of bounds to the OP, but I sure would test the Nikon, best way to rent.
Paul C