Coinbase was founded in 2012 in peak mongodb hype, and while the issues with MongoDB are well understood today, six years ago you'd have to be a cutting edge emerging database type developer to know that, and that's not the type of person who Y Combinator likes to fund.

I don't know; it sure seems like people understood the issues in 2010: https://news.ycombinator.com/item?id=1636198

I still don't understand them past "no PK/FK integrity" and duplicating data + problems with complicated queries.

What's the best current alternative database for storing financial transactions? Just use a SQL over a NoSQL?

NoSQL is a meaningless term, and SQL is a query language. The actual database types are relational, key-value/wide-column, document store, graph, search, OLAP/column-oriented, etc.

Any of them will work as long as you can durably store your data. Relational databases have always implemented ACID and SQL which makes them easy to work with, and they can support almost any data model with decades of updates and tooling which is why they are the default choice unless you specifically have other needs. Newer relational databases can also scale horizontally by applying the same sharding techniques as the other database types, so even that issue is no longer relevant.

PostgreSQL all the way down. I know I'm in the minority here.

I doubt you're the minority TBH.

Probably OneTick.

If you subscribe to the Lean Startup learning where the author spent 2 years building out a product and then realized that nobody wanted to use it the heroku + mongoDB stack makes a lot of sense. It lets a startup to get a product out the door quick and figure out product market fit.