read the thread there, and there were some interesting points, I was thinking however more of complete purpose made factories, which avoid some of the contentions over there. specifically: due to increasing use of robotics, the production of assembly-ready parts in a factory would only require a design, and a little computer / fabrication / mechanics skill, some high-yeild processes might even go from high-quality input materials to finished product with no human interaction (after the process is initiated).
Difficulties would include:
purpose made equipment may only be good for 1 thing. (the less you have control over the process the less options and difficulty you have.)
manufacture of the high-quality input materials.
power generation.
in some circumstances, synchronization of a process may be very difficult or impossible on your own
an additional point from over there that was shot down quickly for seemingly little reason was increasing prevalence of 3D printers.
one of the negatives for a prototyping machine that was given was wasted materials, but current 3d printing wastes relatively little.
the most prevalent material for 3d printing is plastic, but metals, foods, and even organic material are starting to be used.
The most common method of printing is a melted plastic extrusion process, yet some use photosensitive materials and a projection system or lasers to harden it. metal printers can use a laser sintering process or electron beam sintering to make finished products (not just prototypes anymore) even titanium aircraft parts are being made this way.
the fanciest ones are of course very expensive, big, and rare. they only require relatively low ability compared to traditional manufacturing however, and are very versatile.