Actually the only language I used autocomplete was Java, mainly because it's almost impossible to work in Java without a IDE. However sometimes I turned autocomplete off because those long menus are a pain to see. I don't use a real autocomplete in Emacs with Ruby, Python, Elixir, JavaScript. I use pabbrev which remembers the words I type. It's usually enough not to make typos and it helps to remember the methods/functions.