My first project at Google was working on fake reviews on Maps. ML was certainly useful for some things, but at the end of the day sometimes you literally have no practical way of knowing if a review is fake or not. Was this written by a customer or by the business owner's brother in law? Or by his competitor down the street? Who knows? Which means it's pretty hard to get good, complete training data to train your model on.
Of course, there are classes of fake reviews that are easy enough to detect. But as the fake review-writing AIs get better, I don't know how the anti-fake review AIs can win.