It's a bad fit if you have enough compute work to keep all the CPUs busy. Then you're dealing with thread priorities, infinite overtaking and starvation, fairness, and related issues.
I have an unusual application, a metaverse client for big 3D worlds. It has to deal with a flood of data while maintaining a 60 FPS frame rate. It's essential that the rendering thread(s) not be delayed, even though other background threads are compute bound dealing with a flood of incoming 3D assets. This does not fit well with the async model.
This sort of thing comes up in games, real time control, and robotics, but is not something often seen in web-related software.
If you create a library that uses async, you're forcing everybody that uses the library into async as well (with the same executor).
If somebody writes a library now that's generally useful but uses async, it forces others to use async or rewrite the library themselves.
On the one hand a lot of people put this down as whining about free code, which is somewhat true, but the infectious nature makes the whole ecosystem less useful if you want to build something non-async.
