relay to you the experience ive had with 3 (lol) 8kx's, all have been ok lens and display wise. no ipd issues etc. getting that bit right was as simple as other hmd's for me. but i understand what you're saying as i've read about numerous reports similar to what you say. i don't understand why there is such variance, unless there is variance between pimax hmd's, but as i mentioned the 3 ive had were ok.
I wonder is it something to do with the price of the headset? That they put more effort into the production and quality control of the higher end headsets. And most of the variance in quality happens in their cheaper line of headsets like the 5k models.
It's very face shape dependent - I have an average sized head with average IPD (almost to the mm), so within Pimax's stated specs I should have been fine - they also said the 8KX would ship with face foams for different face shapes, mine came with one, the thinnest one obviously for a flatter wider face shape than mine. I found that with the pimax set to the lowest IPD setting, I had to shift the headset left to get my right eye in the sweetspot and that then meant my left eye was all blurry and vice versa, I tried all the different hardware and software adjustments suggested but no matter what I could never get both eyes in the sweetspot and it caused raging headaches within 15-20 minutes of use. When I took off the headset and measured the physical distance between the rings in the lenses that would be the sweetspot, they were 66mm apart, despite the software telling me the minimum IPD of the headset was 60mm - other people had measured and found the same discrepancy and were having the same issue.
The 5K, 8K and 8KX all had the same issue for the same types of people.
Apparently the early prototypes didn't have this issue and the lenses were also praised as being excellent and Pimax deliberately switched to a lens which was cheaper to produce and the guys that were involved in reviewing / testing the release candidate did tell Pimax about this issue and they ignored it because they wanted to use the cheaper lenses.
They also had more than one headset (one of the earlier 4k ones and the 8K) where they stated the headset had a particular resolution (e.g. full 4K RGB panels) but they were actually less than full RGB so had less subpixels / resolution than advertised.
So its not variance in their headsets that is the issue, its just a design flaw that means that the headset isn't to its advertised specs, if you don't need 60mm IPD or you have a wider flatter face then it might not affect you, but if you are buying a Chinese developed product on the assumption that it will fit a European face shape or come with the specs its advertised with then it can cause you a big issue / make it not usable as you'd expect.
I haven't had any similar issues with DK1, DK2, CV1, Quest 2, Vive, HP G2 (all of which I've owned) or any of the other headsets I've tried, so quite why every other headset maker manages to get IPD settings correct for a wider range of faces and just pimax had it wrong, I don't know.