All of that is true, yet PointFree's version is usually reliable right away, with some tests needing some megaYield tuning in CI. So I'd rather have API that is 99% testable rather than 0% testable. But you're right, we fundamentally need core concurrency testing features, like the ability to wait for the runtime to finish all work, or to add probes or barriers to know when certain sets of work have completed. Really, any testing support would be good, as right now there's nothing at all.
(I believe the sleep nonisolated(nonsending) issue was recently discovered and discussed here, so perhaps we'll see some changes.)