Gallegos - Wasn't even a notable prospect at the time he was with the Yankees. He had good stuff but no command. Who knows if he even develops if he stayed with the Yankees.
Sonny Gray - Unfortunately some guys just can't pitch in NY. We've seen it with numerous pitchers regardless of who the manager or pitching coach is.
Kaprelian - If Gray wasn't a headcase this could've been a good trade. Kaep looked done up until this season as he's never been healthy, and the other two prospects (Fowler and Mateo) are non-factors
Ottavino - This was essentially the equivalent of letting a free agent walk. He looks good so far but let's see how he finishes the season, and if Boston makes the playoffs good luck with him on the mound
Whitlock - This one hurts, but Boston took a flyer on a guy who was injured and nobody had seen pitch in awhile, and he's worked out. Would like this one back.
Good farm systems produce good prospects, and you can't always hang onto every one. Boston doesn't seem to lose out on a lot of prospects, but then again they haven't produced many good players over the past decade (at least on the pitching side).
And if you're dinging BC for trading good pitchers, then he should also get credit for trading for Green/Cessa, picking up Loaisiga/Luetge off the street, and signing Chapman/Britton/Cole. No GM has an unblemished track record.