It looks like [`WP_Comment_Query()`][1] only supports a single post type.

You can adjust it, by using the `comments_clauses` filter. 

Try for example:

    $defaults = array(
        'number'    => 5,
        'post_type' => array( 'post','authors','movies' ),
    );
    add_filter( 'comments_clauses', 'wpse_121051', 10, 2 );		
    $comments = get_comments($defaults)

where

     /**
      * Support for multiple post types for comments
      *
      * @param array $clauses
      * @param object $wpqc WP_Comment_Query
      * @return array $clauses
      */   
     function wpse_121051( $clauses, $wpqc )
     {
    	global $wpdb;
    	
        // Remove the comments_clauses filter, we don't need it anymore. 
    	remove_filter( current_filter(), __FUNCTION__ );
    
    	// Add the multiple post type support.
        if( isset( $wpqc->query_vars['post_type'][0] ) )
        {

            $join = join( "', '", array_map( 'esc_sql', $wpqc->query_vars['post_type'] ) );
 
            $from = "$wpdb->posts.post_type = '" . $wpqc->query_vars['post_type'][0] . "'";                         
            $to   = sprintf( "$wpdb->posts.post_type IN ( '%s' ) ", $join );

    	    $clauses['where'] = str_replace( $from, $to, $clauses['where'] );
        }  
  
    	return $clauses;
     }


The plugin:
-------

As kindly suggested by @kaiser, I made a [small plugin ][2] to add multiple post types support to `WP_Comment_Query()` and `get_comments()`. Let's hope this missing feature will be supported by the WordPress core in the near future ;-)


  [1]: http://core.trac.wordpress.org/browser/tags/3.7.1/src/wp-includes/comment.php#L165
  [2]: https://github.com/birgire/wp-comments-from-mulitple-post-types