The first time I found FoundationDB, what impressed me was the tuple structure that they use to store data.
However, after reading more about it, I realized that FoundationDB is really a system that you can use to build all sort of databases (although it can become quite complex depending on what you want, but I guess it's still easier than building a database system from scratch).
You can make your own API that fits your needs or use FoundationDB's layers. Apple is using a record layer for their CloudKit.
I might experiment on it on a personal project, you know, to kill an ant with a bazooka.