Remember, you always remember your busts but you don't think of another teams busts. Some Sox fans might be familiar with Henry Owens a few years from now but 99.99% of baseball fans will not know who he is.
I think there are so many variables at work here and part of it is just pure luck, but we've done much better than normal at developing positional players and that is probably partially due to luck, in this case, good luck as opposed to bad on the pitching side. But part of the reason for having a farm system is to be able to trade them for needs. We might not have developed many of the pitchers on our team but we've deployed assets to build some of the best pitching in the league.
Also, just because guys like Henry Owens failed, don't mean that guys like Groome and Mata will fail either. Also, I believe it is worth noting that Henry Owens was never regarded as a high ceiling prospect, to begin with.