The frustrating truth about being an SEO, or anyone striving to fully comprehend something, is that we often know something works without understanding why.
We notice, for instance, that better UX correlates with better keyword rankings.
Linkless mentions, or a brand appearing on the internet without a link to their site, have been eluded to as important by Gary Illyes, Google’s Webmaster Trends Analyst.
Does having positive sentiment, or customers saying good things about your company, also help? Does a proper reading level for your content, matched for an audience, improve your page algorithmically?
Another way to think about these unknowns is that they matter to humans. A well-organized & beautiful design, a brand valued everywhere, and clear writing could only make a good impression, leading to more traffic and backlinks which certainly do matter.
So while those fascinated with a subject will never rest until finding greater certainty, we can at least say that these factors, directly or indirectly, will improve SEO.