What Miller is saying here is that the very values that made the Puritans so capable of doing bad things (like the witch trials) also made them capable of doing great things (like surviving and prospering in this fairly harsh place where they had come to live.)
Miller says that the Puritans' no-nonsense attitudes really helped them survive and thrive. They did not waste any of their energy on being fancy or on having arguments over who would lead them or anything like that. Instead, they obeyed their leaders and they worked hard. This sort of single-mindedness allowed them to overcome great obstacles. At the same time, though, it also led them to be fairly intolerant of those who did not act and think "right."