Not every postgresql installation can perform every postrgresql feature (via plugins). That doesn't make postgresql incompatible with itself.

I use a postgresql connector to execute some postgresql specific sql statement on CockroachDB. That's a baseline quality for "compatible".

We do have compatibility gaps -- some big, some small. But I would still call it compatible because it's definitely close enough to use a PostgreSQL driver with it in production. I would be curious to hear your opinion on which incompatibilities are most important to address.