The main downside I can see to a book being made into a movie is that movies can't leave in all the details. For example, in Lord of the Rings: Fellowship of the Ring, quite a few details about the time between Bilbo's birthday party and Frodo's leaving are left out in the movie, such as Frodo selling Bilbo's house. This is because it's very difficult to fit the content of an entire book into between 90 minutes and 2 hours, meaning a lot of minor plot points and scenes must be removed to fit in the time frame. This can cause a few plotholes in the movies that aren't present in the books.
These plotholes can stop the watcher from getting fully immersed in the story. Books let the reader's imagination run wild and let them picture for themselves how characters would look, scenes would play out, and areas would look. It creates an entire movie-like sequence in the reader's head that matches their tastes. An example of this is when I was reading Divergent. Though I haven't watched the movie yet, one part of the book - I won't disclose which, for spoilers - made me stop reading in anxiety for a few moments, something that is very difficult to pull off. I could feel the same feelings as the main character herself, making the entire scene much more engaging.
Sometimes being too engaged in a book can be exhausting, which is why taking a break can be very helpful. Unlike while watching a movie, where it's difficult to bring yourself to pause the movie to do something else, you can read a book for as long as you need, then put it down while keeping your page. No one can sit down and read an entire 150-page novel in one go, right? Unless your mind is completely set to finishing it all at once, which can break the immersion, having a break is crucial to the reading of a book, but a disadvantage to watching a movie.
In conclusion, I believe that books are far better than movies. This is because they're more contentful, immersing, and pausable. So next time you go to watch a movie that was made from a book, read the book afterwards and try to see how much better it is.