What are some common factors that affect GA aircraft manufacturability?
This answer can only scratch the surface - a good answer would take you several days to read.

Manufacturability is the combination of several things which combine to 


improve the repeatability of the finished products without much individual reworking, and
reduce the work hours required to produce a given component or system


under the local conditions (wage level, worker skills, energy prices, logistics etc.). This list of factors is incomplete but should give you an idea what is involved.

This is done by selecting the proper techniques (lots of jigs and molds for repeatability, the fewest amounts of parts and joints for reducing man hours) and by designing the system such that producing the components and combining them into the finished product can be done quickly and with the available workforce.

An example: When Fairchild joined up with Dornier, they planned to relocate the production of the Do-328 from Bavaria to Arizona. The production line in Germany was set up for the local workforce, and since they were highly skilled metal workers, the holes they drilled manually were straight within ±2°. In the USA, however, Fairchild could not find enough adequately trained workers, so the production line would had needed jigs for every single hand-drilled hole - and there were lots of them on the 328. The whole endeavor floundered because the production line was not designed for manufacturability under the new local conditions, and remedying this would had cost too much.

Another example would be the Tu-4. This was a Russian copy of the Boeing B-29, which the Russians had ample time to study after some landed in Eastern Siberia after attacks on Japan. But it was no cheap knock-off: The Tupolev design bureau redid the design, airframe, engines, systems and all, in metric dimensions, so standard Russian gage sizes, fasteners and tools could be used. It was re-designed for manufacturing, in other words.