I did not pitch at a high level, but between baseball and softball, I did pitch for many years. It's hard to explain precise reasons, but from my experience, the final 3 outs are the most difficult to get. They are somehow different. The pressure is far more intense in that final inning of a close game. Focus levels are increased. Batters don't want to make the final out. Some umpires shrink their zone right at the end which is tremendously frustrating. I think that even as robotic and refined as MLB players are that pressure and added focus make that last inning different. MLB players have such a highly refined degree of muscle memory that thinking doesn't really enter into the equation very much. Yogi Berra was famous for saying that you can't hit and think at the same time. In that final inning, thinking becomes a bigger part of the game, and that is where the problems probably start.
I don't like that Aceves has said that he is not comfortable in the role. To me, that is an acknowledgment that he is approaching it differently than he has approached his other roles in which he has been very successful. He has felt it necessary to dial up his velocity, but it has changed the movement and command of his pitches. He's not used to pitching at that velocity. He may start to learn how to pitch at that speed, or he may find it difficult to adjust. If he dials it back down, that may not work either. Hopefully, he can get it figured out quickly, because I don't see much possibility of another closer coming to the Sox.