I think async creates a mini-language within _any_ language, i.e. the whole what-color-is-your-function problem. It's turtles all the way down or nothing.

In return, of course, you get a style of concurrency that tends to be much easier to reason about and much less prone to subtle bugs than traditional preemptive multitasking.

Whether that tradeoff is worth it is obviously very dependent on the particular situation.