I think memory is a limiting factor mainly for certain kinds of novels - crime/mystery ones, definitely, where the author's both got to recall what's going on everywhere, and remember what they've actually told the reader. Perhaps also for major serieses like Discworld, or Harry Potter (although that's a bit short) - the need to maintain consistency between books, particularly if you've got an obsessive fanbase who'll notice if, in book six, some peripheral character had four sisters and now, in book twenty, he's got three... If you're writing some postmodern standalone novel then you can forget all sorts of things and leave lots of plot-ends un-tied-up, and that's fine!