There are synchronous human tasks in some assembly-lines, but as long as they're rote tasks, inefficiency usually isn't introduced. Humans working as if they were machines, are rarely inefficient; and if they are, this inefficiency is "legible" for blue-collar work in a way that it isn't for white-collar work, so an inefficient human "part" can be, er, swapped out.
The whole "thing" that Toyota did to revolutionize car manufacturing, was essentially to make quality-assurance part of the same ground-level machine (i.e. make it a computed outcome of a series of "dumb" machine and worker steps), rather than making it a separate auditing process. As such, they essentially squeezed the ability to be inefficiency/incompetent out of the QA process.
