The only thing DynamoDB can do good is simplicity. Except for this, even MongoDB has tons more features than DynamoDB and the new version resolved performance problems existed in the previous versions.
Apples and oranges. People who call tools like this garbage fail to evaluate trade-offs at their required complexity space. Distributed systems have extreme trade-offs. More features => more bugs.
MongoDB really? You are comparing a software product to a multi-region global data service? This is just not a great comparison. If you build a distributed global data service on MongoDB you could compare that to DynamoDB.