@Chris Anderson, all valid questions. The application is extremely light weight. At this point, we are only overriding 2 functions to introduce the functionality. Having said that, as IPB changes and updates are made, this makes it extremely easy to update alongside IPB. IPB devs has already done the leg work in developing the application to be easily extended. This is how they have already integrated Sendgrid, SMTP, PHPMailer etc. These are all extensions of their base Email class. To be honest, I am surprised they have not included many other email service options as it was very easy to build in this functionality.
Second, this application is built on AWS's open source PHP SDK which is heavily supported and has a very large audience.
Third, we have a fairly large audience in the applications we've developed with over 1000 clients having used our services.
Lastly, and in all reality, I wouldn't be too worried. I am confident IPB will introduce this functionality natively in the future as they already work with S3. All it takes is for a community to rely heavily on a third party application for them to introduce it into their core application. Thanks for the questions!