I’m still pretty much a complete noob. If you’d asked me a few weeks ago about UV Lightmaps I’d have guessed you were talking about about some sort of appliance for getting a tan.
So please keep in mind that what I mention below might be based on misunderstandings of what UE4 does.
When a static mesh FBX is imported and that FBX does not have a second UV map for Lightmap is there any chance that static mesh will look correct once everything is built/baked? If not, why not issue a warning when that FBX is imported?
Creating a static mesh from brushes… Again, if the mesh won’t work unless there’s a second UV map for lighting why not have UE4 offer to create one right then and there utilizing the “Generate Unique UVs” functionality?
If the “Generate Unique UVs” doesn’t do a good job unless certain values are used then perhaps those values should be supplied as defaults.
W/R/T the steps outlined here… https://answers.unrealengine/questions/29092/convert-brush-to-mesh-now-i-have-overlapping-uvs-h.html
…Perhaps UE4 should offer to perform those steps automatically?
If some time could be spent making Generate Unique UVs do a better job I’m in favor of that. 
If “Generate Unique UVs” is always going to produce suboptimal results then perhaps UE4 should immediately prod the user to go add the lightmap UV channel when the user either
- creates a static mesh from brush, or,
- imports an FBX that doesn’t have the lightmap channel.
When it doesn’t have a 2nd uv channel, the ue4 uses the 1st one as a lightmap -> this can cause overlapping uv errors which will look pretty bad after you have build the light. So for complex meshes it is crucial that you generate a 2nd uv channel with lightmap in your 3d program (it just takes some few sec. to do this :)) but for example a cube wont need one.
So there is a chance that it will look correct without a 2nd uv channel, but as I mentioned, just for simple models
It’s not likely a mesh won’t need a second UV channel, sometimes there are cases where your UV’s already follow the guidelines for lightmaps, in which case you don’t need to.
The reason it doesn’t tell you when you might need lightmap UV’s is because you might want to use the first UV channel for your lightmaps in which case you don’t have a second channel, or you don’t intend on using lightmaps. Even in some cases even if it gives you an error about your lightmaps they might still turn out to be fine–that’s cases where for instance I set them up what I think is correct but there might be a small error somewhere that it’s detecting but it still renders fine.
Generating lightmap UV’s in UE4 doesn’t usually give a good result, it’s just an option if you don’t care about spending the time to do them properly.
Thanks for your thoughts and replies guys. Seems like this stuff is a bit more squishy than I thought.
Lesson learned: Just because an import or brush to mesh conversion succeeded doesn’t mean that the resultant asset will render behave as one might expect.