But that could only ever be worthwhile in markets with very few competitors. But when there is only a low number of competitors, customers usually have better ways to form an opinion than internet reviews.

The only exceptions are cases where customers are faced with a wide range of vendors, but only tiny, well-delineated subsets are in actual competition. Restaurants serving travelers in a small town would be an example.

While this is true, being so specific about something negative that someone calls them out is much more risky than the useless 1 star rating with comment "product did not work as expected". Low quality comments are almost never considered valid by those that pour over the comments.