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it looks a bit dated
All of his stuff is. "Dracula Unbound" reads like it was written in 1950, not 1990. The Helliconia trilogy feels to me the least dated of his work, managing some character development and complex interpersonal relationships that his other work mostly lacks. And the scope that @AlexD refers to is much more ambitious (and successful) than the kind of "Big SF" done by Arthur C. Clarke or Greg Bear (who mostly just do "Hey, look! I'm describing something really fucking big!").
I have part 1 of helliconia, it looks a bit dated, but we will see