Second and most notable thing, performance. One of the reasons people like sql databases is strong consistency for writes. That means that if you send a write request and right after that a read request for same data you would instantly get what you wrote. Beforementioned solutions by big companies usually sacrifice that for performance. In short, for each request you would need to propagate data across all the nodes responsible for hosting the users data, and you'd need to have consensus protocol. Thats all slow. Expected response time for query is in milliseconds not seconds.
I hope you find a solution for this, if you do be sure to message me and I'll be the first one to use your product. I'm also sure you'll get a big fat paycheck by one of the big companies for that solution.
Good luck.
In addition to the nosql solutions you mentioned, there are also SQL solutions directly. For example
Amazon RDS
https://aws.amazon.com/tr/rds/
Azure SQL
https://azure.microsoft.com/en-us/services/azure-sql/
Google Cloud SQL
https://cloud.google.com/sql
I know very well the difficulty of the subject I am talking about. You are talking about the consistency of the select query following Insert/update from a different peer. As I said at the beginning of the topic, I believe we can do it and I wonder if it is worth the effort.
I'm well aware of those, but that services are not marketed as extremely scalable solutions. Yeah its worth the effort if you can make that. If you crack this and make a distributed sql storage that responds in milliseconds, you'll be a millionaire easily. No one has been able to do that so far. Thats the reason big guys invented nosql, so it can scale easily across many instances.
Consistency is just one of the issues, your sql db needs to be acid compliant otherwise, there is no point in using it. You can’t just reuse mysql/postgres for this, you need to make your own db.