"Nothing", in principle. But they're bug factories in practice. It's really easy to throw "past" cleanup code in a language where manual resource management remains the norm.
It's not that they can't be used productively. It's that they probably do more harm than good on balance. And I think async mania is getting there. It was a revelation when node showed it to us 15 years ago. But it's evolved in bad directions, IMHO.
Yeah node showed that a native async single threaded runtime can be performant. Seems like this knowledge was lost to the world somewhere along windows vista; everyone who has had to ever develop in the cooperative world of early winapi could tell you that easily.
.NET wasn't the first either. Lisps were doing continuations in the 70's.
But "invented" and "revealed" are different verbs for a reason. The release of node.js and it's pervasively async architecture changed the way a lot of people thought about how to write code. For the better in a few ways. But the resulting attempt to shoe-horn the paradigm into legacy and emerging environments that demanded it live in a shared ecosystem with traditional "blocking" primitives and imperative paradigms has been a mess.
I think you're underestimating the role of .NET in this. It was .NET that popularized this concept for the masses, and from there it spread to other languages including JavaScript, which also borrowed the exact same async/await keywords from C#.
