It looks like `WP_Comment_Query()` 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 $c WP_Comment_Query
      * @return array $clauses
      */   
     function wpse_121051( $clauses, $c )
     {
    	global $wpdb;
    	
        // remove the comments_clauses filter, we don't need it now 
    	remove_filter( current_filter(), __FUNCTION__ );
    
    	// add the multiple post type support
        if( count( $c->query_vars['post_type'] ) > 0 )
        {
            $from = "$wpdb->posts.post_type = '" . $c->query_vars['post_type'][0] . "'";
            $to   = "$wpdb->posts.post_type IN ( '" . join( "', '", $c->query_vars['post_type'] ) . "' ) ";
    	    $clauses['where'] = str_replace( $from, $to, $clauses['where'] );
        }  
  
    	return $clauses;
     }