RethinkDB's ReQL is a very powerful functional query language. The functional aspects of ReQL and the straightforward implementation of the Node driver for Rethinkdb make it a natural fit for Javascript developers. You no longer have to type some obscure syntax in quotes (aka SQL), your queries are just "natural" Javascript functions in the same way you would use lodash to handle your collections.
Cassandra is very scalable and achieves the highest throughput for the maximum number of nodes compared to other alternatives. Unfortunately this also brings rather high write and read latencies.
After a very promising start, development began to drag after major supporters like Canonical, Selenium and CouchOne either shut down or moved to other tools. Development has begun to pick up again.
It is easy to replicate a CouchDB instance to another, either as a snapshot or as a continuous replication process. In addition to hosting locally, you could replicate to one of the commercial CouchDB providers like Cloudant.
Since CouchDB is considered an AP (Available, Partition-Tolerant database management system), it is not really consistent (not all clients can have the same view of the data consistently) and the only way to achieve some "eventual consistency" is through replication and verification of data.
You can extend ArangoDB using JavaScript that runs directly on the Server (Google V8). You can build data-centric microservices that aggregate, validate, transform or enrich data (It's up to you, plain JavaScript) and provide those via a custom API route.
Concourse tracks versions of your data by default. This way you can easily edit data without being afraid of losing anything since Concourse can easily revert to a previous state.
ConcourseDB is known to have a very holistic approach to robustness and data integrity which is reflected by it being fully ACID compliant.
ConcourseDB has always been strict about making sure data is valid before allowing it into the database, and there is no way for a client to bypass those checks.
Since ConcourseDB is not a very popular solution among developers, it may be hard to find libraries or SDKs to easily integrate it into any platform with which you are developing.
Realtime data change notifications are blazingly fast which makes synchronization between apps instant and effortless. When used offline, local and remote changes are synchronized upon reconnect without any hassles. Changes are logged on the property level which makes them synchronize extremely efficient and fast.