I'm not aware of any specifically. It could be a very niche area within a particular fandom. Since 'rated M' implies mature content, it might not be as widely popular or publicly accessible as other fanfiction.
It's difficult to say. 'Hunhan' is not a widely known mainstream term in the general fanfiction world. And the 'rated M' aspect limits its audience. There may be some within the dedicated 'hunhan' fan community, but it's not likely to be something that has a large following across all fanfiction readers. Maybe if there is a very active 'hunhan' community, they might have some such works shared privately among themselves.
There may be some in the depths of certain fanfiction communities. However, because it's related to 'hunhan' which is not a common global fanfiction topic, and also because it's rated M so has restricted access and a more limited target audience. It's not likely to be among the most popular fanfiction out there. Most popular fanfiction usually has a broader appeal and is more family - friendly or at least not restricted to a mature - only audience.