There are generally two classes for FDWs: Postgres<->Postgres and Postgres->Everything else.

The first one is generally suitable for production and is very useful for sharded Postgres when you want to communicate across shards without having to go back out through the application.

The second one's mileage really varies. Some implementations might or might not be prod ready or mig target only specific version combinations. Can be very useful for data engineering or analytics use cases for quick ETL into a staging database. Or for data migrations between database vendors.