Releases go through different testing channels before landing in stable (testing > alpha > beta > stable). Usually, announced releases eventually land in stable, but testing is there to catch severe bugs and regressions. "Buggy" releases are therefore discarded before they ever land in stable.
Seamless updates work very well, and there are no staged rollouts for OTA updates. By default, the updater checks for updates every 4 hours, and it can be triggered manually. If there's an update, it will be fetched and installed right away. If it fails for some reason, it will keep retrying and you'll get notified by an error notification.