For me it is irritating as the editor need to reload edited file after each save, which sometimes screw the undo (I have persistent undo in Vim enabled). Additionally it fires file watchers twice, so sometimes it can cause “wasted cycles” when running tests twice after each change.

Additionally mix format will not work with files that aren’t UTF-8. While most editors use that by default it not always can be a case, for example Windows sometimes use UTF-16 which will crash the mix format:

$ echo 'Łukasz' | iconv -f UTF-8 -t UTF-16 | mix format -
mix format failed for stdin
** (UnicodeConversionError) invalid encoding starting at <<254, 255, 1, 65, 0, 117, 0, 107, 0, 97, 0, 115, 0, 122, 0, 10>>
    (elixir 1.10.0) lib/string.ex:2223: String.to_charlist/1
    (elixir 1.10.0) lib/code/formatter.ex:198: Code.Formatter.to_algebra/2
    (elixir 1.10.0) lib/code/formatter.ex:236: Code.Formatter.to_algebra!/2
    (elixir 1.10.0) lib/code.ex:645: Code.format_string!/2
    (mix 1.10.0) lib/mix/tasks/format.ex:418: Mix.Tasks.Format.format_file/2
    (elixir 1.10.0) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (elixir 1.10.0) lib/task/supervised.ex:35: Task.Supervised.reply/5
    (stdlib 3.10) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
$ echo 'Łukasz' | iconv -f UTF-8 -t CP1250 | mix format -
mix format failed for stdin
** (UnicodeConversionError) invalid encoding starting at <<163, 117, 107, 97, 115, 122, 10>>
    (elixir 1.10.0) lib/string.ex:2223: String.to_charlist/1
    (elixir 1.10.0) lib/code/formatter.ex:198: Code.Formatter.to_algebra/2
    (elixir 1.10.0) lib/code/formatter.ex:236: Code.Formatter.to_algebra!/2
    (elixir 1.10.0) lib/code.ex:645: Code.format_string!/2
    (mix 1.10.0) lib/mix/tasks/format.ex:418: Mix.Tasks.Format.format_file/2
    (elixir 1.10.0) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
    (elixir 1.10.0) lib/task/supervised.ex:35: Task.Supervised.reply/5
    (stdlib 3.10) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

Yes, but it will not manage all files in projects. Erlang files or Makefiles will not be formatted via mix format. And if you configure tabstop option then GitHub will also show files in diffs and file display with proper indentation depth.

What is more, some editors (for sure Vim, but I assume that others as well) can display vertical line on the “line length limit” which is useful to write code “formatted” already, without the need for running mix format in the background. None of the editors I know can read .formatter.exs to do so (however I could easily implement that for Vim if needed).

The for me in EditorConfig is to make it pass before even being sent to the CI, as most of these “fixes” can be applied automatically by editor (proper indentation characters, indentation width, charsets, etc.). This is also handy for root = true option for a lot of tools, as this marks the “main” directory of the project and such tools can “stop” when traversing the directory tree in lookup for “root directory”.

I marked yes, although I don’t use it in all my projects (or even most of them). But I like having it and want to use it in more projects. Even if it is only as documentation about the expected config.

It is more about using it anywhere. I understand that most people will not use it everywhere, because sometimes it doesn’t make sense, because internal projects not always need them.