All nodes validate the root hash of every shard, so no, a Sybil attack on one shard is no easier than on the entire blockchain. It may even be harder, as there

All nodes validate the root hash of every shard, so no, a Sybil attack on one shard is no easier than on the entire blockchain. It may even be harder, as there would be more nodes in total when the hardware/network resources a person needs to run a node decline from what's needed to validate the whole blockchain, to what's needed validate the fraction of the blockchain contained in one shard, or conversely, when 64X more economic activity, and thus more people, are utilizing the blockchain, as a result of the scalability boost provided by dividing the blockchain into 64 shards. >>Also, if you have, say, 10 shards - that isn't giving you 10x throughput. Its gonna give you maybe 2x throughput because what shards are doing is allowing lower-power machines help run the network, and those lower-power machines are not as effective as the rest, so they aren't contributing as much. They don't have to be lower power machines. If the blockchain is divided into 64 shards, then the total network throughput can increase by 64X with the power requirements of validating machines not changing.