What do you think about Firebase Database?

As I've mentioned in some posts, I have created and used a lot of databases at work (DB2 z/OS and MS SQL Server) and in my projects with MySQL. The last days I took a deeper look at some posts here about FB DB and read some examples on the www.

So far I understood that it's all about storing JSON trees in a flat structure (nesting can be is bad). So it's more like a well known list with maps. To me (with the lack of deeper knowledge) it seems to be very tricky to build a good structure/db-design. Handling data can be very hard, too. So what do you think?

I don't know, I'm old enough to know DB2

And I still have to read also this (a little bit out of topic):
https://www.facebook.com/notes/facebook-engineering/tao-the-power-of-the-graph/10151525983993920

With strong SQL (DB2, MS, Sybase, SQLite, ...) backround for me it was difficult to understand concepts of FB as well. After reading some examples and after own tests I started to understand.