And OS X -- it's hardy bad by any stretch. Mostly it's fantastic. There are the rare glitches, but as other commenters have said, there are several factors at work, what with increasing complexity, efforts to get platforms to work more closely together, proliferation of multicore CPUs, 64-bit, evolution of security and networking technologies, bringing Swift and modern programming language patterns into the technology stack, dealing with changes in unstable third party platforms, and preparing for whatever unknown things Apple has in the oven.
Apple is trying to bring the future to us. Doing this kind of work, over multiple versions with plans spanning years or even decades, is not simple. This kind of work has been likened to rebuilding a 747 aircraft while it's flying. IMHO it's probably even harder than that, and Apple is doing a damn good job so far.
Admittedly there's a lot of assertion here, from my own experience of things working just fine, and not much (even anecdotal) evidence, but I just wanted to say one needs to view the small bugs in the perspective of all the significant work that is being undertaken.