Presentation of theory in textbooks is a matter of both content and style. It is common to find that two students with the same teacher disagree about whether the teacher is clear / understandable, for example. The teacher may be presenting at a level that suits one student but bores another, or confuses a third. The teacher may be presenting in a way that engages one student while another student's eyes glaze over. The teacher may choose to approach a topic from a basis where one student is strong but another is weak. The same issues arise with textbooks.
The Cambridge books (Advanced and MX1) are very strongly influenced by the style and approach of lead author Bill Pender. I found him to be an amazing educator but not everyone liked his approach and equally not all will find his explanations suitable. David Sadler, lead author of the Cambridge MX2 book, was (in my opinion and when I had each of them as a teacher) the stronger of the two in providing explanations for students who were struggling. Dr Pender was at his best with a strong / high-performing class who he could extend with challenging materials. Oddly, however, Mr Sadler was better at writing challenging exams. Looking back at SGS papers, those written by Mr Sadler always have challenging and interesting questions at the end, though neither wrote exams that were easy.
Given that the population of students here at BoS skews to the more able, I am not surprised that Cambridge books are preferred for both questions and theory. That does not mean that MiF can't be a better book for explanations for some students, though I agree that the questions in MiF don't cater well to more able students. MiF's style does not suit me but I would not have a concern if a student of mine preferred it as their reference for theory and understanding.