It's about Roundcube's public_html directory, not web server's public html directory. In other words, now the entry point(s) (index.php file) is not in Rouncube's root, but /public_html. And there are other files than index.php.
Do you mean that we would need to setup a forward from the website root?
mail.example.com/ -> mail.example.com/public_html/index.php
While not a technical difficulty, it does make for an ugly URL. Does it have to be named "public_html"?