The general advice should be: use PostgreSQL in case you are uncertain what to use. Watch some Youtube video's with Michael Stonebreaker (2014 Turing Award winner) and start getting disillusioned by the NoSQL hype.

Then, try to understand the mess Edgar Codd tried to fix in the '60s and '70s.

Do you have any specific videos we should watch?

In the top-hit on Youtube, Michael starts to discuss the solution-space @31m15s (https://www.youtube.com/watch?feature=player_detailpage&v=OY...)

You can write a DDP backend which is backed by Postgres. In such a case it ought to feel like Mongo is just serving materialized views of the genuine, consistent data. If you treat the Minimongo data that way—just consistent enough to show an image once—and verify all the writes on the server then you ought to be able to get by.

I think Meteor is great, except for that one thing. I won't be touching it again until there if full support for one of the SQL technologies.

Have a look at ToroDB (https://github.com/torodb/torodb). It's open source, MongoDB-compatible database which uses PostgreSQL to store data. In a relational way (i.e., no jsonb, no blobs). It's still under heavy development, but worth a look (ToroDB dev here)

Simply put, make sure your app can handle inconsistent data.