The absolute worst part of OS autocomplete in general is the myriad of apps which some-fn-how break typing wherever an autocompletion is available.

I have an autocompletion for "aa" and it's triggered before I press space, meaning it's impossible to type Aaron in these fields.

I can only imagine the pain of using "composed" input methods like CJK, etc where every glyph requires multiple keypresses: https://en.wikipedia.org/wiki/Input_method