I don't think FTP can work like a storage engine. Because a storage must provide direct URLs to files. With FTP this is possible only by exposing access credentials and making it possible to access any data on a FTP server. Also I don't think that for example video players could properly work with FTP links. 

So I believe for such a case it'd better to connect FTP share on a lower level - at a hosting's OS level to not expose FTP credentials and make it possible to seamlessly work with it as if it were a local storage.