the remaining 10% of the 90%-off free lunch is pretty much just eventual consistency - it can occasionally be the case that you write something to the DB, and an immediate subsequent write doesn't see it. That said, there are escape-hatches there (we'll proxy queries that happen inside of a transaction to the upstream mysql/postgresql database, and there's an experimental implementation of opt-in Read-Your-Writes consistency), and I'd wager that the vast majority of "traditional" web applications can tolerate slightly stale reads.
Our official docs also have an aptly-titled: "what's the catch?" section: https://docs.readyset.io/concepts/overview#whats-the-catch