Took a quick look at the code, because I definitely remember reading the smoothing groups. Ran into a "This is a confusing mess for a lot of users causing them to complain about weirdness, so let's just calculate our own and be done with the whole thing" comment, so...
Yeah.
Maybe if you can give me like a couple of "here's some vmfs, with images of what I want these things to look like" examples, I can do some kind of "no, really, I want to use the smoothing groups" override option down the line? Maybe.
In my specific use case, I generally want most of the map geometry to just use flat shading, but then I'll have something like a pipe or column where I want the sides to be smooth. This would be very easy with smoothing groups, since I'd only need to use them on these specific brushes. Right now, I just have a 40 degree threshold set, so stuff like pipes gets smoothed automatically, but then some angled walls get smoothed as well when I don't want them to. There is a way to bypass this by having the faces of the wall spread between different objects, but situations where this is a practical workaround are very rare.
I generally assumed that there's probably some major hurdle with the smoothing, so I'm not surprised that there's no simple solution. But this is essentially the one feature that I think the plugin could still benefit from, since the lack of control over the shading is a pretty noticeable limitation. And yeah, I know I could just export the meshes to Blender and adjust the shading there, but this kind of defeats the purpose of using HammUEr in the first place. Everything else can be done using just Hammer and the UE editor and this would be the one outlier.