ISFJs are considered to be conscientious and methodical persons who value harmony and cooperation. This article says about the strengths of ISFJ personality type. Strengths and weaknesses come from traits of particular personality type.
ISTJs are people focusing on building long-term, stable careers. So, career choices of ISTJ personality type includes career which give stability and security. Mostly they would not prefer jobs that are not stable or secure. They also prefer to do jobs to be in line with their principles and follow traditional rules.