Elizabeth Bear has an excellent technical explanation as to why many people have been saying that Ian McDonald’s Brasyl is “too difficult”.
I, of course, love “difficult” books. I’m a Gene Wolfe fan. But not everyone likes to have to work hard when they are reading, and each author has to make a decision as to what sort of books they want to write. Typically a “difficult” book will get you critical acclaim but lower sales. A “difficult” book is more likely to win a juried award than a popular vote award. And so on.
But you know, the world would be boring if everyone wrote like Gene Wolfe, or everyone wrote like Naomi Novik.