Exactly what do the robots do? The are used to make parts like machining castings, punching and molding rubber and plastic and electronic bits. These parts get made by vendors and are installed on assembly lines.
As for the line of cars, robots weld steel frames but the Model S is a glued aluminum car. Robots stamp steels of metal -probably a lot fewer of them on a CF body. Robots paint cars better than humans. Even Ferrari prefers robot painting.
Robots have nothing to do with a poor quality car. They are tools like anything else. On the line the Robotic or CNC programmer/operator, the Quality Control department checking the work, and the management team that supports perfection is what makes a problem free item