A MongoDB shard isn't necessarily a single-point-of-failure since a shard is usually deployed as a replica set. If a shard's primary node goes down, a secondary

A MongoDB shard isn't necessarily a single-point-of-failure since a shard is usually deployed as a replica set. If a shard's primary node goes down, a secondary node in the replica set is elected as a primary and takes reads + writes. Similar to what you mentioned for Scylla - a node can go offline on a shard in a MongoDB cluster and it keeps humming.