Ted Williams said, probably more than once, that the most difficult skill in all sports, not just baseball, was being able to hit a round ball with a round bat squarely.
Me, I think pitching is a really close second because command of pitches is so important, and you have do that in addition to having great stuff and throwing so hard you put your arm at risk (Tommy John surgery is edging toward preventive surgery for little leaguers these days).
The above to me translates into a simple reality that it is not easy to build a great bullpen before the season starts. So far this season, short as it is, Carson Smith looks awful and rookie Poyner looks pretty darn good (1 runs in 4.2 innings, most by any Sox reliever), mostly because of his location (in my view). Kimbrel, our $13M closer, has pitched four innings and in three of them had two, two, and three baserunners before getting the third out.
Right now I'm pretty happy with this bullpen even though it's the rotation that has been so insanely good. And it is unarguable that the pitching staff is carrying this team.