One of the big challenges mentioned with promises is that you have to kind of commit to promises linking to promises... this is a common problem with async systems added later, where you have to "line them up like gears", and you can't just do async functions which call non-async functions which call async functions and expect it to work. Python has this problem too.

There's a solution in delimited continuations, however delimited continuations seem to only be used and understood in the Scheme community (are they used anywhere else?). Delimited continuations allow you to suspend your code to a "prompt" lower in the stack at that point... and it doesn't matter if you have non-"async" code in between.

It'll be nice when they make their way to other more mainstream languages.