Review books are, well what their name implies they are, review books. Class notes/syllabi will usually be much more detailed than what is contained in your standard review books like FA, BRS, or Kaplan. Review books are designed to paint the big picture concepts, which the USMLE expects you to know. To answer your question about review books being in depth enough for Step 1, the answer would be, your basic science medical education from the first two years + the well rated review book is good enough depth coverage for Step 1. So if you've passed all your classes, then reading the review book will be enough for Step 1. Remember you cannot possibly remember everything. Once you finish year 2, once you go through a review book, your memory will be refreshed and you will say ahhh, I remember that or ahhh I had trouble with this back in year 1 or 2. Hence the review book has served it's purpose. Now on the other hand, if you are just simply using a review book with no background at all, you may run into problems with knowledge gaps. The authors of these review books assume that you know or at least have covered some of the details while in med school so they choose to omit them, hence why your review books won't have all the details of your class notes/syllabi/texts.
