Here's a fascinating article over on Royals Review. It asks,

based on historical precedent, what can we expect from various kinds of highly regarded prospects?

Of course this is highly relevant to the Royals, who have an extraordinary number of highly ranked prospects in Baseball America's eyes. It's also relevant to the Pirates, of course, since they are planning to compete by "flooding the system" with prospects. Scott McKinney wrote the article, and he termed players with an average Wins Above Replacement (WAR) 1.5 or less as "busts" and those above that as "successes."

The conclusion is that top 20 position players succeeded about 60% of the time and top 20 pitchers succeeded 37% of the time. It's a risky business, especially for pitchers. If one just mechanically applied these rates to the Pirates prospects you would forecast that Taillon (11) has a 40% chance of success, Sanchez (46) has a 32% chance, and Allie (79) has an 18% chance. Comforting.