Soroush Khanlou (via Marcel Weiher):
All of these nuances in the API cause KVO to embody what is known as a pit of failure rather than a pit of success. The pit of success is a concept that Jeff Atwood talks about. APIs should be designed so that they guide you into using them successfully. The should give you hints as to how to use them, even if they don’t explain why you should use them in that particular way.
KVO does none of those things. If you don’t understand the subtleties in the parameters, or if you forget any of the details in implementation (which I did, and only noticed because I went back to my code to reference it while writing this blogpost), you can cause horrible unintended behaviors, such as infinite loops, crashes, and ignored KVO notifications.
I wish Cocoa didn’t have APIs that require you to use KVO.