This website (as far as I know, I don't know if they changed that) since the beginning uses flat files instead of a database. Depending on the case it might actually work and be a valid strategy for persistency
I also think flat file storage is a valid solution. If you are abke to use advanced filesystem features like snapshots and online replications, the use-cases for databases shrink even further.
I built a content pipeline and CMS on top of flat files (json) and mercurial.
You can get repository level transactions by committing on each consistent state (squashing before push) this lets you do undo/redo and recover from failed operations easily - databases sort of give you this - but only for the stuff they actually contain (so unless you're putting your asset files in to the DB you don't get this).
Yeah I have no doubt that flat file are a valid solution, I think the only headache was related to a dupe bug.
"Server-boundary crossing edge-cases and race conditions persisted for a long time -- allowing for gold and items to be duplicated, though I think we had largely eradicated the big ones by the time I had moved on to UO2."
