Sure! Simply put, they do not require enough of the reader. This is true for two reasons. First, the Red Books fail to dive deep enough into the science. Now, they're certainly a great overview (and on the spectrum from most basic to most intricate, they're certainly closer to most intricate than Asimov's work), but their depth is quite different from what a physics student would study. Second, it's impossible to understand physics in the same way that a physics student would if you're not actually doing physics. In other words, you need to do "homework," i.e. exercises; with pretty much any STEM subject, and especially physics/math, you need to get your hands dirty! This is usually done by reading a textbook. So, the Red Books and Understanding Physics are great for enrichment, but by themselves cannot satisfy the level of the understanding that a physics student would have. In fact, the Red Books are so good that I've seen a few upper-level physics classes list them under the recommended reading of the course syllabus.
Note: If you've gone through a physics program, or know a lot about it, then just skip this. Fortunately for any aspiring autodidact, the physics curriculum at most schools follows a similar path. First, the student will begin with engineering physics and calculus (unless he/she places out of them via AP credit; also note that the curriculum I describe is for USA programs only). There's a standard calculus text that most schools use nowadays, and that is James Stewart's Calculus. The engineering physics book is usually titled something like "Physics for Scientists and Engineers." There's like four or five of these books that most schools will pick from. My favorite is Haliday/Resnick's or Giancoli's. After calculus, the student will usually take three additional math classes (generally more if they wish to go to graduate school). The two obvious ones are linear algebra and differential equations. The third one is a course that applies mathematics to physics specifically. This course exists mainly to prepare students for the physics classes that come after engineering physics. Two pretty common books used for this are Mathematical Methods in the Physical Sciences by Boas and Shankar's book. After that, the student will generally take six courses: 2 classical mechanics courses, 2 EM courses, and 2 quantum mechanics courses. The rest of the student's program will be filled in with electives (and some required labs, which you obviously cannot really do outside of school, and probably a thermal/stat. mech course). Now, there's two really cool things about those courses. The first is that most universities use the same textbooks for each respective course sequence. Those textbooks are: Taylor's Classical Mechanics, Griffiths Introduction to Electrodynamics, and Griffith's Introduction to Quantum Mechanics. The second cool thing is that those books don't really presuppose any knowledge of physics. When it says introductory, it means introductory. Of course, they presuppose that the reader has knowledge of linear algebra, differential equations, and a lot of the stuff learned in the math of physicists class (like vector calculus). The two course course-sequences will generally cover the entirety of each book.
So, as you can see, there's a lot of material that a physics student will know by graduation. It's quite clear, then, that the Red Books couldn't possible cover all of that material, yet they remain a great tool for enrichment and exploration of physics.
Now time for one of my favorite Feynman stories, a story that, like most Feynman stories, nobody really knows is true or not:
| Caltech uses an honor system and the exams are take-home exams. The instructions for the exam read “You have three hours. You may use your class notes and Feynman [referring to the Red Books; it's customary to call textbooks by the author's last name rather than the title].” The student took the exam to Feynman’s office, and he agreed that the instructions included him as a valid resource. Feynman completed the exam in half an hour and the student got a perfect score.