I think I had the same confusion after reading the GP's last statement. The idea of hashing is that the resulting number will be uniformly distributed across the hash space, as opposed to monotonically increasing bit by bit like an auto-increment or timestamp, but it's not the partition key value itself that needs to be uniformly distributed but your access patterns of the partitions. As the docs note, a device id (and also a user id) could be good or bad to use as a partition key, it depends on the read/write patterns given the key.
