Adding to @automatico’s feedback, lots of people have used AC in a transactional manner, utilizing Ecommerce Deep Data, Event Tracking and event start triggers, and personalization tags. Additionally, you can create and send a campaign via API.
One important thing to note though, is that these will still be marketing emails conducted transactionally. That means that there exists a possibility that a contact can unsubscribe from future communications, unlike “pure” transactional systems like sendgrid/SNS etc…
This is where it makes a lot of sense to have specific lists, so that if a contact does unsubscribe, they are only unsubbing from a few emails. I would think pretty hard about using in-app messaging (payment issues, password reset emails) in AC, but I would certainly handle all behavioral marketing messages in AC driven by events.
Let me know if you would like me to expand on any of that!