My pork sausage patty recipe requires very precise timing between flips and temperature changes to come out like I want so it is clutch here for instance. I had to reverse engineer it watching someone make it otherwise it would have been lost to time, instead it is archived for all-time thanks to my autistic archiving of all our family recipes collected over the ages. It would have been an insane pain in the ass if I was not competent at programming already.
 Specifically the mdutil family is not optimal at minimum and it routinely forgets manually began indices. Finder's copying algorithm is fubared for very large file transfers despite OS X shipping with rsync which correctly solves this problem trivially for the past decades. Time Machine is fubared in that it can result in corrupted archives that go unaudited. Automator and Apple Events remains largely untouched despite being the most compelling reason to use OS X and being best in class anywhere, other efforts on other OS's have been abandoned unfortunately. iOS and OS X's security model is not uniform despite iOS having a much better implementation. HFS+ is completely fucked as a filesystem in modern contexts. Dtrace has remained untouched since its implementation in OS X and is subject to bit rot.
Safari's archival system is not open sourced and not homogeneous for no compelling reason. iOS needs a unified task management/notes/appointment system, there is tremendous overlap between Notes, Calendar, Mail, and Reminders that has extensive obvious points to merge functionality rather than maintain separate implementations.
Spotlight behaves significantly different between OS X and iOS, granted these differences are not obvious and difficult to track but have resulted in having to manually track information at various points in time, however in the tractable column we can formally verify the implementations are exact which is the correct way to solve this and should be the workflow in every project in modern development, see the prevalent innumeracy amongst developers for why this is difficult to achieve though.
It's so goddamn useful though that everyone should be hustling to become competent in this area and is extremely worth the time investment for young programmers in particular.