What exactly is millennial writing?

For the context: recently I started hearing this term more and more often, in relation to books and games. At first, I thought that this is inspired by Marvel's movies and the way they are written, but some reviewers sometimes give examples of oxymorons (like dangerous smile, deafening silence, etc), calling them millennial and therefore bad. I even heard that some people cannot read T Kingfisher books as her characters are too millennial. So now, I am curious what does it even mean, what is it? Is it all humour in book bad, or am I missing something?