(Disclaimer: Max from ArangoDB here)

The purpose of this benchmark series was not to provide a comprehensive test of all these databases. We only wanted to demonstrate that a multi-model database can successfully compete with specialised solutions like document stores and specialised graph databases.

I agree to your comment about graph databases, the crucial thing is that the neighbors of a vertex can be found in time proportional to their number, and that the queries involving an a priori unknown number of steps (graph traversals, path matching, shortest path, etc.) run efficiently in the database server and can be accessed conveniently from the query language.