Having been through a through a number of textbooks, I can say without reservation Jackson is one of the best books I have studied. Some books are simply incomprehensible (leaving out details in the derivations, omitting crucial facts, speaking in confusing language, speaking unclearly). Some books I get the feeling are written by people who are probably great physicists, but awful theoreticians, or at least, awful expositors of theory. This isn't surprising considering that physics is a field involved with much 'intuition' and not everyone is expected to be great at dealing with the theory. On the other hand, the book by Jackson is so comprehensible, it's simply a masterpiece.
Other books are comprehensible, but they don't teach you how to solve the problems, by leaving out too many examples, or having things too abstract, or phrasing problems too far away from the material covered. Jackson actually teaches you how to solve the problems. In each chapter, he gives you everything you need to go through and really figure out how to do the work. There are many books which don't manage to meet this common ground. Furthermore, in Jackson many (or most) of the problems have the solutions written down as well, which can be indispensable for learning.
On the downside, Jackson is not easy to skim through. If you want to look up something, it takes like 1-2 hours IMO to carefully read through the relevant sections before you can feel satisfied. But the fact that you can sit down with the book, put time into it, and really learn an answer or a solution to a deep question... it's a masterwork, like I said.
In short, I think you may have a wrong impression about this.