if you want really fast quick-and-dirty UVs without much distortion, just in order to be able to use Substance Painter, I would recommend using MODO's atlas projection feature.
That's just one click for each object, then you pack the uvs and you are done.
IMHO this UV mapping is far from perfect (it wastes space), but it might be good enough - the distortion is minimized
I do this all the time in order to get stuff into Substance Painter quickly.


Here you see 4 different sub objects (from a tutorial for MOI that I am working through right now)


Now I do the atlas projection, just one click (while all objects are selected) and we get UVs for everything, but they are overlapping


Now this is the pack dialog


And this is your UV map.


If you up the rez for your textures you might get acceptable results.

Of course you can create multiple UV maps with this method, for each group of objects that you want to handle in Substance Painter individually.

I do not know if Blender or any other OSS has a comparable feature.

Cheers
PS Sorry I wanted to reply to the main thread, not the last answer