In the sheet metal shop I'm employed in, there used to be a few jobs that we used threaded inserts for (AKA "PEM" and "clinch nut"). Lately we haven't used them much.
They do require proper equipment (High tonnage press of some sort, or PEMserter) to install them properly, but once installed they do work quite well. I'd recommend that in a small home shop you stick to drill/tap or tack welding a nut on the back of thinner stock.
Not sure about the round coupler nuts... not sure that really makes sense, but can't offer a qualified opinion on that front.

-Sparky