The authors of this protocol made a valiant effort to defend against sybil attacks by requiring intersecting quorums. However, it's not clear to me that they succeeded. Specifically, I worry that they are depending on people running nodes to vet other nodes in some out of band fashion. Clever and patient sybil attackers could insert themselves into the network over weeks or months, and then disrupt it while shorting it on exchanges, or by conducting double spend attacks against exchanges.
As was shown in Bitshares, which relied on holders of BTS to vote on "good" block producers, users can not be relied upon to make these judgements. They will either vote at random, or vote based on trivial stats like uptime. The holders of BTS ultimately paid the price when the creators of the coin forced through a proposal to increase the supply cap and the price collapsed as a result, but the creators have since moved on to other coins.
It seems to me that proof of work works, and proof of stake (as implemented in Tezos) may work, although it's not been running successfully for very long. I'm very suspicious of other consensus algorithms protecting billions of $s worth of assets.