Yes - good question. Actually I used Lambda+DynamoDb because I really wanted to get some hands on experience with them. I don't think DynamoDb was the best choice though - it's great for fast inserts and reading a limited set of the newest items, but not that great for aggregations/statistics (like how much money I've spent in the last three months). It's purely a cost issue - I need to provision a lot of RCUs if I want to do these kind of statistics when I have a lot of users. And RCUs cost money. I like the manged nature of it and that I get replication for free but it all comes at a price :)
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