Everything is in the ledger, just like the blockchain.
I've already went through this with another Ripple supporter
here:
Integrity of the total XRP count is equivalent to the integrity of the ledger, like the blockchain. Validation is performed by few parties today, and that is legit ongoing grounds for criticism. As the network grows, that will become less of a concern. Harming the integrity of the ledger is not in the interest of current Validators or Ripple Labs, as it damages the network. You can learn more about the Validator consensus process
here.
The integrity of Ripple is based on the assumption that nodes do not collude.
The problem with this is that as they say in all disclaimers: "past performance is no guarantee for future returns."
True, that's why it is very important to choose well the nodes in your UNL.
If you add official public validators run by academic institutions in different countries, non-profits, government bodies of non-allied nations such as China, Russia, India, Pakistan, Iran and USA, competing private companies of different countries, libertarian groups, activist groups like EFF and Wikileaks, hacker groups, and competing political parties, and ... they manage to collude in such a way that more than 50% of them agree on the same lie knowing that their fellony is going to become history in the next 10s... the whole world is in an unspeakable shit that far exceeds immediate concerns about Ripple's integrity...
POW secured convergence of the ledger in Bitcoin is in contrast an objective proof of the will of the economic majority.
POW is weak because economic interest leads to the pooling of computer power between the hands of a few major players like mining pools and ASIC makers in the case of Bitcoin. Instead of having reputable organization of all nations and all walks of life puting their reputation at stake to collude, all you need in Bitcoin is to have a handful of Chinese ASIC makers and mining pools colluding.