I'm not talking about trade-offs of async vs sync. I'm talking about the color article saying languages have "color" when one type of functions (sync) can't use results of another type of functions (async). If you have threads and mutexes, then one type can use the other, regardless of whether that's a good idea or not. The point is that there either is a hard unsolvable api-infecting limitation (like in JS) or there isn't.