I think High Sierra is the worst Mac OS release yet. I’m sure there’s a cognitive bias partially to blame (since it’s the most recent) but it looks like we are way past that.
The mach kernels in general are buggy, regardless of the release. e.g. I'm pretty sure they end up delivering SIGPIPE to the wrong thread in some circumstances on Sierra. There is also a problem with recvmsg not returning control messages some time. Another reason they killed OS X Server maybe.
Remember Apple started down the "server" road decades ago with their A/UX UNIX server (https://en.wikipedia.org/wiki/A/UX). Throughout the 1990s you could get a high-end Mac spec'd as a UNIX machine if you wanted, though I've only ever seen a handful in the wild.
The introduction of a special-purpose server was an anomaly, the Xserve had no specific predecessor, perhaps some kind of "go big or go home" effort on Apple's part.
While that system wasn't bad by the standards of the time, it couldn't compete with the likes of Dell, HP and others who offered way more flexibility on configuration and who would seemingly sell super budget low-end servers at a loss to lock people into their product line so they can squeeze them on support costs, a model Apple's allergic to.