The real difference is where the rules live. SQL puts them in the database, so a bad write fails immediately. Document stores put them in your application code, so the same bad write succeeds and you find it months later when a report will not add up.
Firebase, MongoDB and DynamoDB are all reasonable for the problems they were built for. The mistake is picking one because it was quicker to start with, then discovering your data has fifteen shapes and nothing to enforce which of them is current.
Updated September 2026