Yes, the "downside" is that data needs to be collected. We have a couple of data points which are available from the blockchain, but to get good results we need to store more, site-usage specific sets.
As we're all committed to the idea of self-ownership, this will be made transparent though. Users will have to opt-in, and then will get an overview of what data we store about them, fine grained control over what they don't want to be stored in the future, and of course the possibility to delete it themselves without it still remaining in some hidden database.