Tried it in a side project, very cool. If you, like me, build a small app like a To-do list it is nice because you do not need a database server opened. Also used firebase Auth and this blew my mind how easy it is to bring those "connect with Google" feature.
I guess I would not rely on it for a project with a complex schema because I still have the feeling storing my key pairs is volatile (no foreign keys, no constraints, I prefer relational databases).
I do not know at this date if they dispose of a relational store, but if they could come close to Google Cloud SQL, I think they would engage more developers to use this great tool.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.