I'm not sure if Snowden particularly knows what he's talking about in this case really. I mean, the 51% attack is hardly something most don't know about.

http://cointelegraph.com/news/115133/edward-snowden-on-bitcoin-bitcoin-by-itself-is-flawed

“Obviously, Bitcoin by itself is flawed. The protocol has a lot of weaknesses and transaction sides and a lot of weaknesses that structurally make it vulnerable to people who are trying to own 50 percent of the network and so on and so forth.”

'Despite his concern for the digital currency, Snowden believes the concept of tokenization and proof of work could be implemented to create “some very interesting things,”...”