I have been using API2Cart since beginning of 2014 for our new webstores product. We have successfully integrated close to 100 stores and are currently doing 5 new stores every week. We started with the standard plan (shared hosting) and in 2015 we upgraded to a dedicated server to increase the number of monthly and concurrent API requests.
As an ecommerce company with more than 20,000 online sellers security is very important for us. During the time that we have used API2Cart we have not had any problems related to security. The support that API2Cart provide us with is excellent. Communication is quick and the expertise from their engineers have helped us to successfully integrate API2Cart with our own platform.