I'm surprised it hasn't been mentioned here, since this is a thread comparing Bitcoin to Ripple... isn't it a big drawback that Ripple must depend on UNLs in order to fend off sybil attacks? This UNL thing seems rather cumbersome and centralized.
With UNL, you are trusting people to tell you which nodes you should connect to. I thought one of the main reasons the bitcoin invention was considered so great was because it came up with a way to get around the need for cumbersome UNLs by using proof-of-work, and connecting out to many nodes.
The network is made up reputable parties, and according to the wiki "immune" to Sybil attacks :/