Except when Apple deprecates APIs but the replacement doesn't have close to the same functionality (looking at you screen capture kit).
Or when the documentation just doesn't explain anything and you have to reverse engineer the API to figure out what it does.
or how there's a bunch of hidden APIs only certain vendors are told about so you can't even compete on an even playing field.
And don't get me started on the C, C++, ObjectiveC and now Swift monstrosities. Having fun with your legacy project when the new APIs require swift, so you have to use the objectiveC bridge and the weird bugs that comes with it.