What is also great about RDS and DynamoDB is that they are “managed services”. Amazon manages them, not you. The result is extremely reliable services out of the box, which you can customize so that they basically never go down (distributing across zones and regions).
nderground uses RDS/Postgres for cases where we need reliable (ACID) transactions and DynamoDB for everything else. We store large data entities (images and large amounts of text) on S3.