Two horrible choices of databases... first they used MongoDB then they migrated to Cassandra. I've used tons of databases [1] in production and those two are the worst.
[1] I've used RethinkDB, Postgres, MongoDB, MySQL, Cassandra, CockroachDB, TimescaleDB, SSDB, and others
Im not questioning your intellect, but each database has its own use case for usage. You might be expecting wrong things from Mongo or cassandra.
Using gazillion databases for wrong use cases doesnt mean nothing.
From your child comment, you cooked up an in house solution, it may be best suited for you. But for others it would be horrible too.
If Cassandra is suited for Facebook, I think the problem is with you making a choice to use it for something its not suited for rather than the database itself.
Cassandra has been successfully deployed at many companies. Would you care to provide some insight into your experience and why you consider it one of the worst?
[1] I've used RethinkDB, Postgres, MongoDB, MySQL, Cassandra, CockroachDB, TimescaleDB, SSDB, and others