Indexes are better than tags because they do not include every relevant page number for a given term. To the contrary, tags list all items, which make them difficult to process. tiago forte was against the excessive use of tagging because they could easily be misspelled in note-taking apps.
References
Forte, T. (2015, February 11). Tagging is Broken. Forte Labs. https://fortelabs.co/blog/tagging-is-broken/