When I was first building the project, I didn't totally know what schema I would need. I started on Postgres. But having to constantly do database migrations while I was developing was a pain. RethinkDB was great for development while I was figuring out what schema I needed. But now that the website is live https://sagefy.org , the schema started getting stable.
I'm not using perfect third-normal form, but in many cases I have no query needs there so JSONB makes sense. JSONB columns can actually remove much of the need for NoSQL. The biggest gains from moving were: not having to do foreign key validations myself (yay!) and about 2/3 filesize reduction to the database. My database isn't large enough for performance to matter, but I'm sure it would eventually.
If I were to do this again, I would probably start again with a NoSQL database (maybe Redis or ES, or just files even) until I figured out the schema and then move to Postgres before launching. I'm sure there's smarter people out there who can start with SQL and predict easily what the feature set is going to be be before they start, but I don't usually have that foresight. `ALTER TABLE ...` gets old really quickly.
I'm keeping an eye on CockroachDB too... if they can really make something similar to Postgres but easily scales horizontally... that would be amazing.