nuance seems to be used to indicate that a question is subtle or difficult. I'm not sure if it's a meta tag or not. On the other hand, it's used like hard, and the other connotations that it brings are well-covered by differences and pronunciation.
If the tag can’t work as the only tag on a question, it’s probably a meta-tag. Every tag you use should be able to work, more or less, as the only tag on a question. Meta-tags, like [beginner], [subjective], and [best-practices], are useless by themselves — they tell you nothing at all about the content of the question.