I too was curious about the described nature of this wear and couldn’t understand how it could be such a problem. None of my old Stanley or Bailey planes exhibit this so I had a look at the drawings and description of mechanisms on the Union website (you know, when in doubt look at the print…). It appears they use a much different setup than the single brass nut and captured fork that a typical Stanley or similar has. The Union mechanism is actually two separate nuts, one on top and one underneath, and the fork which runs up and down the screw. To adjust you need to slacken one nut and tighten the other, like a jam nut.
Okay, I saw the problem. That iron fork is going to be rubbing directly on the crests of the threaded rod, very unlike the Stanley arrangement where the fork is captured in the brass nut. So I believe the claim that there could be appreciable wear in the sides of that screw after some years. I guess the real question though is whether having a hardened screw will sufficiently mitigate that problem or will it just transfer all the wear to the softer iron fork which I’m guessing would be a lot harder to come by.
Still an interesting design though, and the claim by Union that there would be near zero backlash in the adjusting as well as zero shift once set is probably true. But overall, maybe not quite the longevity of the Stanley or Bailey design. Nice looking tools though, I don’t own one nor have I ever seen too many around these parts.
-frank