On February 06, 2017 RethinkDB was purchased by the Cloud Native Computing Foundation (CNCF) and contributed to the Linux Foundation. This ensures the project will continue to be developed and will not fade away as other projects have done in the past.
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 measuring the read/write performance values on one server, you can simply calculate how many more servers you should add to the cluster to reach the required performance levels, and scale easily.
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.
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.