Mambo-phpShop is a simple Online-Shop application. But it is surely no "out-of-the-box" killer application for absolutely professional companies. It's not tested in 100% secure environments. It's not meant to be 100% performant.
Be prepared to be confronted with bugs, which have currently not been found yet. mambo-phpShop has an active community and a forum where problems and questions can be discussed. (http://mambo-phpShop.net)