I dunno man. A lot of these characters seem like they have more important ways to spend eternity than boning.
Again, I don't think it's USING Nature chakra that is difficult, it's collecting it for you to use without doing something terrible to your body(like turning into a statue, or going berserk, etc.) that requires some kind of advanced technique.
Which reminds me, given what we know about Nature chakra, (that the better you are at harnessing it, the less animal-like you become), is it safe to assume that Kabuto is actually pretty terrible at it compared to, say, Naruto?