"Prefabbing walls off site is a step backwards in efficiency, but it is often promoted by salesmen."
I'm going to take issue with this statement. Prefabbing walls off-site is a step forward in efficiency. It allows construction to occur in ideal conditions; to whit...
No mud. No wind. No rain. Perfectly flat and square surfaces on which to assemble. No ladders or scaffolding necessary as walls can be assembled flat and then moved by machine as necessary. In a properly-equipped workshop, doors and windows can be installed, insulation can be blown in (or otherwise installed) and the air-tightness taping can be completed before the units leave the shop. This allows for a higher build quality at a lower cost. Materials are only delivered to one location and waste is more easily managed. Offcuts can be stored for re-use meaning that the waste per build is lower.
Once the house is completely prepared, raising can be achieved in very little time and generally at the end of one or two days of raising the house is completely air and water-tight. The internal fitting (electrics, plumbing, plasterboard), if not already completed in the workshop, can now be carried out in comfort.
Fabricating a house on site is a ridiculous idea when the option to do otherwise is available. I speak as someone who has done exactly that!
Other than that small rant... I agree with everything that has been said thus far!