The main problem I've seen with Objective-C is due to the fact that nil has truthiness, so this code is dangerous:

    if (input.isInvalid()) {
      reject()
    } else {
      accept()
    }

If input is accidentally nil, it's an invisible bug.

The other main complaint is when you have a long string of methods unexpectedly turn out nil at the end, it's a pain to figure out which one it was.