Cassandra, and by extension Scylla, are commonly used for workloads you probably wouldn't use MongoDB for at any scale. The primary strength is the ability to reliably scale-out the landing of live data. It supports writing more complex data models than logging systems but is slower and more limited for querying complex data models than e.g. an RDBMS.

Cassandra is commonly used as a sink for complex service telemetry e.g. mobile data from the carrier perspective.