I just briefly skimmed the docs, so my observations may be inaccurate, but I got the impression that "stackful coroutines" are basically green threads (not C++20 coroutines, as the name might hint). Green threads need a dedicated stack per thread, with all that implies, and also I don't find the cute "magical" implicit yielding helpful readability- and comprehension-wise, but that's more a matter of taste.

It is either telling or a glaring oversight (let's assume the latter) that there is no comparison with Seastar, which supports both C++20 coroutines and callbacks, with helpful syntax sugar for latter (coroutines are still nicer though!), has green threads too (which it calls "threads"), and can use io_uring, dpdk, etc. Seastar is free, actively developed, and is used by real software with demanding performance and scaling requirements, so in light of Seastar existing I'd expect any new C++ async framework to approach the task of "selling" itself more seriously than "it's like Go or Python but in C++!".