Here's an anecdote: one my friends worked for a startup that at the height of NoSQL hype (circa 2012) decided to build their product on Cassandra. The data they dealt with was highly relational with hard requirements on consistency. He told me working on a NoSQL database taught him a lot about relational databases, because he scoured the Postgres source code to learn how to implement consistent and durable commits across distributed/sharded nodes.
OK I'll say it... Your friend chose wrong. HORRIBLY wrong. I have a lot of experience with both Postgres and Cassandra and the fact that your friend chose Cassandra despite their data model being relational AND having high consistency requirements shows that they chose Cassandra for the wrong reason(s).
Before you come back saying something about scalability, I've run both databases at way above average scale so what you choose should come down to what you need, which is not what was done in this case (your friend)
I think his comment was mostly about the fact that you can learn a lot about relational databases by reading Postgresql's source. He did say at the height of NoSQL hype. To me, that choice of words gives subtle hints as to the historical context and a disclaimer about the pertinence of that decision. Thank you for your input though, it provides food for thought for the next person mulling over a similar decision.
Here's an anecdote: one my friends worked for a startup that at the height of NoSQL hype (circa 2012) decided to build their product on Cassandra. The data they dealt with was highly relational with hard requirements on consistency. He told me working on a NoSQL database taught him a lot about relational databases, because he scoured the Postgres source code to learn how to implement consistent and durable commits across distributed/sharded nodes.