Well, from what you were saying, it's mostly a problem of making your asynchronous framework work well with exceptions. It is true that you need to do special things for asynchronous programming to work well in C++, be it for exceptions or even the scope-bound lifetime management of C++ in general, which all have a huge impact on the design of your system.

In particular most multi-threaded C++ code is incorrect, not because it is impossible to do it correctly, but because the standard tooling is too low-level, each third-party framework targets a different niche, and people who roll their own tend to hack it together.

I understand Seastar is supposed to do it somewhat correctly, so you could suggest to Mozilla that they switch to that.