I just answered this on Twitter, but I think there are two instances where it's a no-brainer to use DynamoDB:

- High-scale situations where you're worried about performance of a relational database, particularly joins, as it scales.

- If you're using serverless compute (e.g. AWS Lambda or AppSync) where traditional databases don't fit well with the connection model.

That said, you can use DynamoDB for almost every OLTP application. It's just more a matter of personal preference as to whether you want to use a relational database or something like DynamoDB. I pick DynamoDB every time b/c I understand how to use it and like the other benefits (billing model, permissions model, performance characteristics), but I won't say you're wrong if you don't choose it in these other situations.