How accurate is G Analytics' Affinity Category?

Been trying to convert a health-related landing page for a few months. Come to check my analytics stat and discovered the Affinity Category tab. None of the categories indicate a health/fitness interest on my visitors. Maybe that's why conversion is poor. Making adjustments now.

But the affinity category is new to me. What's your experience with it?

It's GA's version of FB's like matching system

It's currently clunky but I'm sure if they apply their Machine Learning/AI technology to it (like they did with RankBrain), it will get better.