Order users by number of posts

You are using wrong approach

You are using the wrong approach for what you are trying to achieve. You are using the same arguments for both WP_Query and WP_User_Query (get_users eventually translates to WP_User_Query). The problem is that WP_Query doesn’t support orderby parameter post_count as one post is always equal to one post and WP_User_Query doesn’t support post_type as the user doesn’t have post type assigned to themselves.

This can’t be achieved using standard functions

In WP_User_Query it’s hardcoded that for user posts count, it looks only for post_type post. To achieve what you are trying to, you need to write a custom database query.

A custom solution

A standalone function to achieve this might look like this (please note I haven’t tested it so there might be a SQL typos or smth).

function get_users_by_post_count( $post_type="post" ) {
    global $wpdb;

    $users = $wpdb->get_results(
        $wpdb->prepare(
            "SELECT {$wpdb->users}.ID, p.post_count
            FROM {$wpdb->users}  
            LEFT JOIN (
                SELECT post_author, COUNT(*) AS post_count 
                FROM {$wpdb->posts} WHERE post_type = %s
                GROUP BY post_author
            ) p ON {$wpdb->users}.id = p.post_author 
            ORDER BY p.post_count",
            $post_type
        )
    );

    return $users;
}

To use it simply call it like:

$users = get_users_by_post_count( 'custom_post_type' );

if ( ! empty( $users ) ) {
   foreach ( $users as $user ) {
       // there you have user ID and post_count
       echo $user->ID . ' has ' . $user->post_count . ' posts';
    }
}