Sure, but I wouldn't exactly call them mature yet. I think MySQL is still much easier to trust than RocksDB. (I don't actually know much about RocksDB, but I remember that LevelDB had a bunch of data corruption issues. Hopefully RocksDB doesn't.)
To quote the famous video[1]: "Relational databases have been around since the fucking 70s and are some of the most mature technology you can find."