Welcome to the wonderful world of quality control :)
Making something is easy, making something with a high repeat accuracy and very low fail percentage is super hard.
I learned this the hard way myself with an IO expander for the Atari ST that I designed. I needed it to drive the lathe/mill controller, the ST simply had too little IO capacity so with a bunch of decoders and latches I was able to drive a much larger number of motors and relays.
The first one worked like a charm. So did the second and the third. But the fourth, we couldn't get it to work no matter what. Part variability led to some signal arriving just that much earlier causing the latches to remember data that was still stale. The reason was - in retrospect - quite simple, the design had assumed that the input to the latches had long stabilized before the latch pulse was sent. But because on the ST the latch pulse took a different route as well there could occur a tricky situation where the output of the buffer driving the data bus on the far side would still be rising. To make things worse, this would not happen repeatably, only with certain patterns of 1's and 0's. In the end, a single buffer in line with the latch pulse caused enough delay to stabilize the input to the latches and that solved the problem.
(Of course not with every board...).
Anyway, that was electronics, this is mechanics but the same principles apply.
Something funny is happening in youtube land, classical music.
There are a whole pile of 'youtube wonders' that can play an enormous variety of material but that have never in their lives done a live concert. It took a long time before one of those miracle people owed up to the recipe: record hundreds, or even thousands of times. Pick the best.
That's the difference between an actual pianist and youtube miracle workers: a pianist can do what they do repeatably, youtube miracle workers would end up being booed out of actual performances.
Repeat accuracy is very hard, mechanics, electronics, playing the piano, the subject hardly matters.
I've been reading all kinds of books and articles about quality control last year and I believe that in a mature market it is the only absolutely critical element in any production process. If all your competitors are doing good QC and you don't you're dead. Everything else can be fixed but without repeat accuracy you'll be fighting a lost battle.
Another way to look at this is that for real world stuff where there is a whole company dependent on the output of the 3D printer the cheap one is too expensive, and the expensive one probably is cheap.
That's because if you were allowed to make a million parts with each and you could take the best part from each set of parts they'd be very close. The 'best' part of those two might even be from the crappy machine. But if you needed just 50000 parts that would simply work your cheap printer (or even a small army of them) would wear out long before completing the order, and that's what makes them expensive.