One thing yield/await does is that if plays nice with existing control-flow mechanisms in the language. You can put them inside a for loop, if statement, try-catch block, etc. With promises all of those have to be reimplemented in a library and all your code needs to be converted to the "promisified" version of things.

The different syntax also prevents you from creating functions that can be reused by both sync and async code.

The big thing promises and other callback-based control flow libraries do is that they fix exception handling in async code (its a PITA to add error handlers in every callback when writing async code by hand). They also let you avoid some of the awkward "pyramid of doom" nesting but thats not really killer feature because you can also achieve that by using tons of named functions.