Since I've been using the RSU on a project recently, here's a not at all posed shot of the unit in typical use.
One of the great benefits of the device is that joins are normally very tidy. Attaching the strip of brass shown above with a conventional iron would require the piece to be tinned on the back and then the hot bit dragged along the sides to encourage heat into the join.
The results would be fine but a bit messy. Nothing that a fibreglass burnisher couldn't take care of but another chore.
The RSU only heats up solder between the positive connection above and the earth - the rest of the kit. Therefore it will only heat the solder on the back of the metal strip.
There are problems of course. Because the heat is very localised, it's possible to burn your way through a piece of metal if the RSU setting is too high or the metal very thin.
I'm never quite so convinced of the joint strength either, but then I used to say that about liquid plastic glue and years of practise have show there isn't a problem there.