For me it is that I can trust the refactoring implicitly, and undo works everywhere just in case.

After XCode did some very creative refactoring one too many times, I simply lost trust in it. Perhaps things have improved in newer versions, but there is still too much missing: completions that are really intelligent, as well as intelligent templates.

With AppCode, it's become a game of how few keystrokes I can type to have it complete. It feels like 10% must of the time. The guesses it makes are great and what I would use most of the time, almost magical.