matrix-org/synapse#12024 raised the issue that, since the images we use for Synapse only support SQLite, we might be missing some pretty bad bugs. In this specific case, Complement shows Synapse's implem as passing whereas it's actually broken for any instance using PostgreSQL (which should be most Synapse instances). It sounds to me like we could really benefit from having Dockerfiles and config files for Synapse that support Postgres.