I have recently moved ACF into mu-plugins on a rather high-traffic site. I’ll let you know about the problems as they arise. I simply put the advanced-custom-fields-pro directory into the wp-content/mu-plugins directory and added this modest file to that directory as well:
$ cat advanced-custom-fields-pro.php
<?php
require_once(__DIR__ . '/advanced-custom-fields-pro/acf.php');
The site in question does not need the update mechanism in place as we update the code on another machine, commit to git, and then deploy on the production webserver.
