More than half the guys rated 5 stars end up getting drafted. 4 star is substantially lower, but still exponentially higher than 2 and 3 star. Using guys like Pitta or Milne as examples of the star system not working is ultimately cherry picking from the thousands and thousands of 2 star guys that never see the field. Because there are so many 2 and 3 star players, there are a lot of examples, but what makes what guys like Milne, Pitta or even Baker Mayfield so remarkable is that it's so rare.
Below is a chart of how many players or each star rating were given in 2014 and % that were drafted by 2018. The 2nd is for the year before
2014 Class - 247Sports Composite
5-star: 33 (57.6%)
4-star: 296 (23.6%)
3-star: 1,541 (6.9%)
2-star: 1,666 (1.1%)
2013 Class - 247Sports Composite
Five-star: 35 (65.7%)
Four-star: 330 (23%)
Three-star: 1,790 (5%)
Two-star: 1,720 (1.4%)