At the time MongoDB's sharding story wasn't great. They've gotten better since, but still have a primary-replica set model that has a single point of failure/failover. Cassandra (and Scylla) are leaderless, peer-to-peer clustering. Any node can go offline and the cluster keeps humming. Cassandra shards per node. Scylla goes beyond that and shards per core.

Cassandra and Scylla also use hinted handoffs so if a node is unavailable temporarily (up to a few hours) you can store "hints" for it when it comes back online. Handy for short admin windows.