And this is the rebuttal: https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2019/p08...
There are downsides to stackful coroutines (peak stack usage for example), but I feed that p1364 was attacking a strawman: first of all it is comparing a solution with builtin compiler support against a pure library implementation, second it is not even comparing against the reference implementation of the competing proposal.
The TL;DR of that sums up my opinions pretty well.
As an aside, I know Rust would be unlikely to implement segmented stacks for fibers, given that they were burned by the performance implications thereof previously.