Use Cassandra when you need real time HA cross datacenter without having to manually fail over

Use Cassandra when you're going to need to grow our database cluster often and don't have tooling to handle resharding

Use Cassandra when you do millions of simple queries (per second), not a handful of complex JOINs

I've used Cassandra at 3 different employers now, and I can't imagine using anything else for many use cases, but there will always be some where it's the wrong choice.