Most autocorrect/autocomplete is malware, and a major potential security and privacy exploit. Personally, I can't stand it when a proprietary program completes my sentences and my thoughts. All the messages you type are fed back to the program. Bots read that shit, and who knows who else reads your messages. I know I sound like a 911 theorist (gimme my tin foil hat back twat) but this is different. Time and time again, major and minor corporations are exposed, and it is well documented that your data are being mined.
Apart from all that shit, autocorrect/autocomplete creates a dependency. I can't even tell you how many times I've seen people completely fail while typing a simple sentence, only 'cause their phone didn't have autocomplete... You forget the English language.