I can't remember the details (I'm stuck on my phone but I'll try and dig it up later if I remember) but we had some issues with certain sections not being to it's liking so code that was fine on one xcode version then broke on upgrade.

Similarly I had to replace how we implement C runtime destructors because Apple deprecated the way to write a destructor in a binary. This wasn't too hard to do, just call atexit from the itanium C++ ABI, but we had basically no warning as far as I'm aware.