I don't see how Scylla / Cassandra are real-time but ClickHouse is not. You can load event data into ClickHouse just as fast as Cassandra. The data are instantly queryable including pre-computed aggregates in materialized views. You can get answers in milliseconds on data that is only a few second old. Real-time marketing is now an important use case for ClickHouse, just to give one example. If the problem is analyzing and reacting to external events, ClickHouse is tough to beat.
How are Scylla / Cassandra better?