Not an expert in game development, but I'd say the issue with C++ coroutines (and 'colored' async functions in general) is that the whole call stack must be written to support that. From a practical perspective, that must in turn be backed by a multithreaded event loop to be useful, which is very difficult to write performantly and correctly. Hence, most people end up using coroutines with something like boost::asio, but you can do that only if your repo allows a 'kitchen sink' library like Boost in the first place.
> that must in turn be backed by a multithreaded event loop to be useful
Why? You can just as well execute all your coroutines on a single thread. Many networking applications are doing fine with just use a single ASIO thread.
Another example: you could write game behavior in C++ coroutines and schedule them on the thread that handles the game logic. If you want to wait for N seconds inside the coroutine, just yield it as a number. When the scheduler resumes a coroutine, it receives the delta time and then reschedules the coroutine accordingly. This is also a common technique in music programming languages to implement musical sequencing (e.g. SuperCollider)
Much of the original motivation for async was for single threaded event loops. Node and Python, for example. In C# it was partly motivated by the way Windows handles a "UI thread": if you're using the native Windows controls, you can only do so from one thread. There's quite a bit of machinery in there (ConfigureAwait) to control whether your async routine is run on the UI thread or on a different worker pool thread.
In a Unity context, the engine provides the main loop and the developer is writing behaviors for game entities.
> but I'd say the issue with C++ coroutines (and 'colored' async functions in general) is that the whole call stack must be written to support that.
You can call a function that makes use of coroutines without worrying about it. That's the core intent of the design.
That is, if you currently use some blocking socket library, we could replace the implementation of that with coroutine based sockets, and everything should still work without other code changes.
