Because of the way Domains use SQL, not every JDBC data source can be used to build a Domain. This has been the case with Cassandra. Here's what the admin guide says about Mongo DB:
MongoDB is a big data architecture based on the NoSQL model that is neither relational nor SQL-based. Jaspersoft provides a connector that allows reports to use MongoDB as a data source. Reports based on a MongoDB data source can be used as Topics that allow users to create Ad Hoc views based on the fields returned by the MongoDB query. However, Domains require relational data sources, and therefore MongoDB data sources cannot be used in Domains.
This would be similar for Cassandra.