MongoDB is a document database. If you use CockroachDB you lose this capability.

> I fail to see how a multi-threaded architecture is a problem with Postgres.

While it's not a fundamental issue, it does have some considerable drawbacks. A lot of the work required to make query processing use multiple cores (9.6, and the upcoming 10 release), would've been a lot easier if there were a shared memory space. There's no portable way to guarantee that shared memory that's allocated after process start can be mapped to the same addresses in different processes, so you've to deal with a relative addressing etc, which both complicates and slows down.

There's also upsides, don't get me wrong. Primarily around robustness in case of failures, but also some around resource isolation.

MongoDB also has some very large customers using it so clearly it isn't a complete disaster. I would chose to use it over PostgreSQL any day of the week (not that there is a clear option regardless).