We (Retool) are going to be doing this very soon and cut our database size by 50%+. And you're exactly right: it's so easy to get started by sticking audits (or other append-only data schema) in an RDBMS, but it quickly becomes a headache and bottleneck.
up until some size the headache from maintaining a separate datastore is bigger. everything should be in the RDBMS until proven otherwise for the sake of simplicity. it's actually amazing how much you can squeeze out of 'old school' databases.
