The question of what level of detail is appropriate depends entirely on the attitude and ability of each individual student. Some students enjoy study and want to dig deeply into the subjects. Others simply want to pass the exams and move on to the flying. Some students find that going into the details enables them to understand the subject more fully. Others are overwhelmed by anything beyond the most superficial coverage.
All of the schools are likely to provide sufficiently detailed material to pass the exams, but only you can decide which level is best for you. BGS students use their online forum to point out errors in the material and to ask questions of their instructors. If you look through some of these questions and the responses provided you may get a better idea of whether their system is best for you.