On the first one, two needle feet broke, the lever that pushes on the uptake lever rotated on its shaft , you name it .....
On the second one the gear rack broke, plus a needle foot. This machine sounded like it was full of gravel right out of the shipping box. Quality control??
The operating parts inside an alum. Boss are made of presscast zink, a non durable (but cheap) solution, as opposed to the cast iron Boss parts which are made of steel. You can figure out yourself the difference between cast zink and steel parts. On the iron Boss you can do a simple modification that makes it even better, by adding a spring to the uptake lever that keeps the top thread in tension during the whole sewing cycle.
My advice, go for a cast iron Boss if you plan to buy one, or even better, save up some money and buy a used motorized leather sewing machine, Juki 441 clone or Adler 205
my 2 cents/ Knut