My team is currently using the hosted cituscloud, which uses PG Bouncer. They note that the reason for sharding is because of high writes. We've actually seen big benefits for moving over to a sharded setup via Citus just as much for the read performance.

By sharding by customer we're able to more effectively leverage Postgres caching and elastically scale. Since switching over, our database has performed and scaled much better than when we were on a single node.

There is some up front migration work. But that's limited some minor patching of activeRecord ORM, and changes to migration scripts. We considered casandra, elastic search, as well as dynamoDB, but the amount of migration changes in the application logic would take an unacceptable amount of time.