I used to be a cheapie bike builder.
I worked for Huffy Service First from 1996-2001. I put together bikes at KMart, Target, Sears, Ames and occasionally at ToysRus and WalMart. I got paid by the piece, anywhere from $2.05 to $3.50 per bike.
I tried to assemble the bikes correctly but it was tempting to try and just slap them together so I could make 4-5 bikes per hour. I usually made 3 bikes per hour. The brakes were always good, the shifters were OK. I usually just made sure that the derailers would make it from high to low without the chain falling off either end.
The biggest detriment to quality assembly was how the bikes were stored. If the bikes were kept vertical they were OK but if they were stored horizontally and stacked on top of each other or leaning to the side they'd have plenty of warped rims and bent derailers .
When I started the big brands were Huffy, Murray and Roadmaster with all 3 making bikes in the US.
Now it's Magna or Pacific with some Huffy but all are made in China.