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' => 'post'
    );
    add_filter( 'comments_clauses', 'wpse_121051');		
    $comments = get_comments($defaults)

where

     /**
      * Support for multiple post types for comments
      *
      * @param array $clauses
      * @return array $clauses
      */   
     function wpse_121051( $clauses )
     {
    	global $wpdb;
    	
        // Adjust this to your likings:
    	$old_post_type  = 'post';
        $new_post_types = array( 'post', 'authors', 'movies' );  
    
        // Remove our 'comments_clauses' filter, since we don't need it anymore
    	remove_filter( current_filter(), __FUNCTION__ );
    
    	// Add support for multiple post types
    	$from = "$wpdb->posts.post_type = '" . $old_post_type . "'";
    	$to   = "$wpdb->posts.post_type IN ( '" . join( "', '", $new_post_types ) . "' ) ";
    	$clauses['where'] = str_replace( $from, $to, $clauses['where'] );

    	return $clauses;
     }