It sounds like you're saying "explicit FSMs are strictly more powerful than promises/coroutines". Which is true. However, they're inherently difficult to program. In my view, the key to simplicity and correctness of programs is reducing the amount of states the program can possibly have. Isolating a piece of code into a blocking sequence seems like a fairly good way of doing it.
Not sure what you mean by "simple examples without error handling". Just like normal synchronous code, promises and coroutines use exceptions, which have the nice property of composability: one catch for several statements/promises. So error handling is the same as in synchronous code.
Now granted, there's a LOT of async/reactive problems where sequential abstractions like promises are irrelevant. GUI programming comes to mind, where you want the view to reflect some reactive state that changes in arbitrary order.
Not sure what you mean by "simple examples without error handling". Just like normal synchronous code, promises and coroutines use exceptions, which have the nice property of composability: one catch for several statements/promises. So error handling is the same as in synchronous code.
Now granted, there's a LOT of async/reactive problems where sequential abstractions like promises are irrelevant. GUI programming comes to mind, where you want the view to reflect some reactive state that changes in arbitrary order.