One of the most difficult books I've ever read is Richard Posner's The Problems of Jurisprudence. Maybe I wasn't as familiar with Posner or legal theory at the time, but I read large chucks without having the slightest clue what he was talking about.
On another note, I was arguing with a friend today about whether shuir (classes we had in Yeshiva) was more difficult than law school. At the end we both agreed that law school is conceptually easier than the more difficult shuirs.
The point of disagreement was whether shuir is more difficult than high level legal theory. Law school is for the most part basic legal thought. It's like the basic shuirs we had when we started Yeshiva. It takes a while to get the thinking down, but once we have it, the concepts aren't difficult.
So here's the question for anyone who has ever done advanced studies in any field: do you feel that Rav Chaim (or any comparable work) is more difficult conceptually than whatever you are studying? Honestly I never attended a really difficult shuir on a consistent basis, but I have learned the more difficult achronim and I don't see them being more difficult if we remove the language barrier and we are able to work through the unclear language.
So what does everyone think?