I'll try to take a stab at it this weekend.
I've been doing some more testing and while I'm still not 100% sure about this, I think my hunch about distance from the map centre being a factor may have been correct. In parts of the map that are closer to the origin, the texture alignment seems way better. It's still a bit off on some really narrow faces, but in 90% or so of cases it looks pretty spot on.
If I copy the same textured brush to different parts of the map, then import them, it seems like the texture offset indeed gets worse the further away it is from the centre. Again, I can't confirm it with complete certainty, but I'm quite convinced this could be the source of the problem.
Sorry, I haven't really had any time to look into it much yet because of Medical Bullshit, but after some thinking (which is unfortunately all I can do right now) and asking around, you're basically right?
So the way UVs work in the older engines is that they're contiguous, which I sort of keep... but which leads to problems in Unreal, because it Does Not Like That.
The solution is apparently to go into the static meshes that have problems and check "use full precision UVs"?
(which is and absolute pain right now because there's no bulk way of doing this, but if that solves it for you, I can see about making this the default)
No problem - this is very important to me, but it is not very urgent at all. I definitely think it should be fixed, but as far as I'm concerned, there's no big hurry - so by all means, please take all the time you need. I hope your health improves soon.
I've tried the "use full precision UVs" option and yep, it fixes ALL the alignment issues I've described - including the warping, so now you can have faces of just about any shape (I haven't tested n-gons much, but all quads and tris are fine now) and everything aligns nicely between different faces and across separate meshes. As far as I can see, as long as this option is selected, the textures in UE match what you see in Hammer with absolute perfection.
Like you said, it's quite a nuisance to apply this to each mesh separately; in my case, it would be feasible, but I'd just... really rather not do it. I understand that applying this option to every single mesh would presumably have some impact on performance/memory, but I think it's absolutely worth it for me. So if you could indeed make HammUEr do this automatically during import, I'd be immensely thankful for it. Maybe it should be optional, but I'd always leave it on anyway.
Give 2.2+ a shot.
There's now an option in the Project Settings/HammUEr page to switch UVs between Default (aka half, like it usually is) or Full Precision.
Note that I had to build this with VS2022 because my machine decided to nuke itself last week (like I needed more bullshit, right?) and I had to rebuild my entire system and pipeline from scratch... but Microsoft doesn't offer non-paying VS2019 versions any longer, so I figured I'd give it a shot.
Should be fine, and Works On My Machine™, but... I guess we'll see.
I've done some quick testing and it looks like everything works the way it should. Checking the new box appears to ensure that nothing is misaligned or warped anywhere. Thanks a lot for adding this and, while I will be sticking to this version, I imagine others may appreciate it if you implemented this in any future releases.