The fact that it's a router and not a PG extension is helpful if you are using cloud hosted postgres and don't want to get into the business of managing backups etc yourselves.

Only azure supports Citus as far as I know.

Vitess is a MySQL cluster manager, it doesn’t support Postgres. https://github.com/vitessio/vitess/issues/7084

SPQR, as the name suggests, is just a query router.