Documentation has been my biggest frustration with Apple development.

Their documentation had been stellar until just around the new focus on Swift. The C, C++, and Objective-C APIs were documented well, complete with guides on the preferred way to use them. I think it might have been well before the Swift switch that they headed downhill - I have a vague memory that some New Big Features came along on OS X and the documentation was lacking and then never caught up to the quality of the older stuff.