DocumentDB more or less is MongoDB though
replyDynamoDB is very expensive.
replyDynamoDB is really a fantastic product for the 99% percentile of workloads that need a particular kind of super low latency read operation at large scale.
replyThe simplicity of using it and the lack of operations is really why most people tend to reach for it over a relational option, although over the years the burden of managing an RDMS has consistently gone down.
If your access patterns are fixed and not going to change, if your schema is pretty much solidified, then yes, I don't doubt that at all. But few things I've worked on have had such stable access patterns or schemas that they would benefit.
replyI enjoyed solving the challenges around using DynamoDB way more than I enjoyed using it on an ongoing basis.