[jordanchan]

AFAIK:

A proper graph db is almost mandatory if there are complex, ad hoc queries that need to be made on real time data.

Using an RDBMS is great if the graph query types are going to be known in advance - so they can be prepared for using materialized views and indexes, and if they aren't too complex - so one doesn't descend into JOIN-hell.

But most applications aren't like that, and not all applications can be completely satisfied using only a graphdb. Hence the rise of the new multimodel databases like OrientDB and ArangoDB. So I think it is a question of what risk one is prepared to tolerate.