Obviously "insufficient" is always going to be subjective. But some technologies really do end up bad by consensus, and I'm getting that smell from async. There really aren't any world class software efforts that rely heavily on async code. Big projects that do end up complaining about maintenance and cognitive hassle, and (c.f. the futurelock thing) are starting to show the strains we saw with C++ exceptions back in the day.

Async looks great in a blog post full of clean examples. It... kinda doesn't in four year old code written by people who've left the project.