The time at which my colleagues found easy ways to lose data was well after Apple had claimed to use it in iCloud at scale. So, I don't think deployment at scale is a proof of correctness. The thing that needs doing is regularly looking in the database for things that should be there.
I don't recall any of those details but the test involved injecting a bogus block device that always returns garbage, and noting that this results in garbage records returned from client queries. And I don't think those kinds of issues have been eradicated, browsing through their github issues there are people trying to recover corrupted clusters. https://github.com/apple/foundationdb/issues/2480
