I feel that nowadays, it's a combination of "things just work" and "if they don't, good luck figuring out why".

I recently installed Tru64 UNIX on a DEC Alpha I got off eBay. I felt like it was more sluggish than it should be, so I looked around at man-Pages about the VM (virtual memory, not virtual machine) subsystem, and was amazed how cleanly and detailed it was described, and what insights I could get about its state. The sys_attrs_vm man-page alone, which just describes every VM-layer tunable, gave a pretty good description of what the VM subsystem does, how each of those tunables affects it, and why you might want to change it.

Nowadays, things are massively complex, underdocumented (or just undocumented), constantly changing, and often inconsistent between sub-parts. Despite thinking that I have both wide and deep knowledge (I'm a low-level code kernel dev), it often takes me ages to figure out the root cause of sometimes even simple problems.