While this is a broad question to ask, in general assembly lines do increase production efficiency. However, there are some instances where using an assembly line is not the best way to manufacture a product, even if it seems like the obvious choice initially.
Assembly lines allow for the quick and efficient assembly of a product, but when it comes to creating the components for said product, an assembly line isn’t really designed to do this. An assembly line only works if it creates products using components that are designed to be fitted together to create a final item. Any changes to this orchestrated process requires that these components are changed before they reach the assembly line.
There are also some added dangers to using assembly lines, for example simple errors can sometimes be missed only to manifest into greater problems later down the line. There is also the loss of individualistic flare and unique artisanship when compared to other types of manufacture.