The biggest challenge yet unconquered is getting your "average" business ($1-100m revenue, zero AI knowledge) using ML to help with literally anything they do . I'd wager less than 1% of businesses outside of SV even have a clue where to begin, or what to use it for, my employer included. "Do we hire some AI guys?" We'll need to crack the 1%-using-it mark for me to consider the revolution "begun"...
this article by evan miller does a good job of explaining why: https://www.evanmiller.org/predictive-analytics.html