iOS is also in pretty good shape, but almost every time Apple releases a new version its buggy. By now, iOS 9 it's a very stable and robust OS, but it needed work up front.
The biggest places were Apple is having trouble are with new products. Watch OS was slow, buggy and limited at release. It's pretty much at a 1.0 state right now. The new Apple TV is by far the best version of the Apple TV, but the OS is buggy and still needs refinement.
My take on this is two-fold:
1) Apple is doing more and more products, causing there to be issues with newer products. They haven't been putting in the QA work on newer software. OS X is old and mature software, so it's pretty stable, but something like Watch OS is very new.
2) Apple's insistence on yearly OS upgrades is causing there to be a lot of 1.0 roughness each year. Just slowing down to a two-year cycle would allow for a lot more time to refine and more time where the OS has been patched and is the latest OS. iOS 10 will be announced in a few months, but iOS 9 still has at least one major point update to go.