Cassandra backup from all nodes or just one?
the short answer -> you have to take snapshots on all nodes.

As you pointed out, Cassandra is a distributed database. As an example, suppose you have 3 nodes with a replication factor (RF) of 2. Each node has primary responsibility for 1/3 of all the tokens in the ring. In addition, each node has a replica from the another node, and nodetool status will show "Owns 66.6%" (2 replicas / 3 nodes). 

If you only backup one node, you only get the data on that node plus whatever replicas are being stored on that node. Since the data is distributed, you will end up missing some data unless you take snapshot on all nodes.