My gut is that sample variation has improved for all the major manufacturers. Once we hit 30MP+ sensors, people started paying much more attention to sample variation. But I haven't really seen anything concrete comparing brands. If anything, it semas like if a lens from a big brand has high sample variation at this point, it's model specific not brand specific.
Dunno whether sample variation actually has decreased - it most probably has.
But it is still way to high.
With all my acquired zoom lenses in recent years (except the Tamron 50-400 where I got lucky) I had to try at least one other copy, often 2, until the amount of decentering or tilt was acceptable.
I bet the camera companies know that most people don't pixel peep so that their calculation is simple: Dealing with some returns is much cheaper than a very meticulous centering control, and thus having a rather high sample variation is a central method to keep lens prices reasonable.
Most high grade zooms are around $3000 if not more so I'd assume people are fairly critical. Mid or lower grade zooms are probably more variable and have fewer mechanical adjustments internally. IME fixed length lenses are more reliable for maintaining alignment.
Overall, I am fairly forgiving of distortion as long as the images look good after correction. Some lenses definitely push into unacceptable territory, though, and it usually shows in the final images.
Unfortunately, no one has figured out how to map Sony distortion profiles to Nikon yet, so distortion does play a factor in some of my lens selections, as severe lens distortion can mess up framing, and especially mess up video. For example I would be happy with the Sigma 20-200mm f6.3 on a Sony camera, but won't buy it to adapt to a Nikon Z camera.
