> It seems like the argument against it is that it has poor ergonomic... but how?

It depends on what you're comparing it against. If you are comparing Rust async against other languages, then the major difference is Rusts borrow checker. In every implementation of async, you effectively move whatever state is needed off the stack and into a separately allocated memory area. In Rust that separate area is a closure.

This interacts badly with Rusts borrow checker. The borrow checker needs to know the lifetime of any object you deal with. It has two "base truths", by which I mean life times it already knows about that you can derive other life times from: static and the stack. If they don't suffice you have to handle life time management yourself and at run time using Rc or Arc or something. Being forced to do that complicates your types and slows the code down. The root cause is async in Rust removes one of those two base truths: the stack. So now you are forced to write that ugly manual life time management code far more often.

This is unique to Rust. Every other language I know of that implements async has garbage collection, so while it remains true they also move stuff off the stack it doesn't change anything. You still use the same types, and apart from sprinkling async's and await's here and there and indenting your closures, your code remains the same.

This is also why I think green threads are a much better fit for Rust than async. Under the hood green threads and async are very similar: they are both ways of doing event driven I/O. Their performance characteristics are near identical. The main difference is while async forces you to move your state to a different area, green threads you do it as before and store it on the stack, just like normal code. In fact green thread code looks identical to normal code. The only change you have to make to convert some code to green threads is change the name of the I/O calls to use non-blocking versions (which is something you also have to do with async, of course). But since the stack is still available all those fights with the borrow checker async creates go away, as does all the extra syntax async requires.

It doesn't come for free of course, so the run time performance of green threads and async is not absolutely identical. In async every task shares the one stack, whereas in green threads they each get a new one. This creates some extra memory overhead, chews up considerable address space (which normally isn't backed by memory) in order to protect against stack overflow, and it costs a bit more to set up a stack. But once a task is setup green threads are going to be bit faster you aren't moving stuff and and off the stack and you don't get hit with those additional run time life time checks you were forced to introduce for async. Mitigating green threads overheads somewhat, a process that is handling 1000's of concurrently connections is unlikely to be running one a machine that is memory constrained, so the extra memory probably doesn't matter. (In reality a Raspberry Pi with 4GB of memory can handle 1000's of stacks.) And it's likely to be a 64 bit machine where address space is nearly free, so the "considerable extra address" space also doesn't matter.

Still, I can think of once place it does matter. You can have two styles of generators: ones take the async approach and ones that take the green thread approach (ie, allocate an extra stack to each generator). Rust nightly does have generators and they currently take the green thread approach(!). But, generators tend to be short lived. You tend to use them to iterate over an array, rather than serving a web request (the typical use a task is put to). That means the overhead of creating the stack for a green thread isn't a small proportion of the time a long running task takes, but could well dominate the time it takes to iterate over a small array. Thus async is a much better fit for iterators.

Currently Rust has this arse about: it has async for long running tasks, and green threads for generators in nightly. With this announcement it looks like this will be 1/2 fixed. Great!