Google Page Rank uses a recursive algorithm because correlation across a high, diverse and strongly connected set of referrers is needed to make spoofing hard enough and thwart sybil attacks. In the case of Steem, the cost of Steem Power is enough to prevent sybil attacks in voting.

There are also potential attack vectors like denial-of-service attack based on leveraging the cycle detection algorithm from a linear increase of controlled accounts, or vote "double spend" as a side effect of reallocating delegated voting power just after a graph rebalancing.

More details in this post