Ohi, author here. The index you build can indeed contain sensitive data, but you have the ability to specify URL patterns to skip the indexing of the matching pages.
Personally, I'd love this is if it were opt-in. That way, I could gradually reduce my repeat search dependence based on me recognizing my actual habits, rather than giving a browser extension carte blanche access to my entire search history. Maybe that's already possible, but I didn't see any documentation about the config file.
sure all data collected is of value to someone, browsing history is definitely. however this is decentralized, and needs targeted attacks, so depending on your threat model, this might be bad, but for most of the users it's probably better than giving a search engine your search queries for pages you visited earlier.