I think that the problem is, while you have near-total control over the information you collect, and can carefully consider its interactions, you have no control over the interaction of your information with other publicly available information. For example, the famous AOL de-anonymisation (https://techcrunch.com/2006/08/06/aol-proudly-releases-massi...) did not (I think it is accurate to say) rely on any metadata attached to the queries, only to the queries themselves.