Some of the stuff they've done isn't necessarily cringeworthy, but it is fairly indicative that they're not machinists. Which isn't an insult or anything, it's actually quite interesting to see how somebody with an entirely different technical background tries to solve a problem.
It's actually a very cool build, although they're spending WAY too much time worrying about a relatively small error- what's they say, 7 thou of sag in the ways? The way the lathe works, that simply means the tool, at worst, drops 0.007" below center- I can't recall the exact math, but there was an example posted on one of the machinist boards, that showed that something like a .020" sag equated to something like a .00025" error at the OD of the work. Might have been even less than that for a larger diameter part.
They'd have been much better off just finishing assembling the lathe, and putting it to work. I understand the need for "new material" for the video channel- that big fancy house and workshop doesn't pay for itself - but surely there's more worthwhile things to work on than that.
(Says the guy with a whole shopful of windmills he's tilting at. )