…NeXTstep was built on mach and, although i’m unsure if any antecedents remain in macOS, it was certainly production-ready in its day; i remember a couple of decades ago there were stopgap versions of the HURD built on top of mach instead of their own microkernel but i thought that was only ever intended as a temporary workaround…
…i presume on that basis that sustained developer interest was its greatest hurdle, no pun intended…