This might make me sound like a bad guy, but there are very few things I wouldn't be able to write. I have a high tolerance and can stomach almost anything, even if I don't like it.
I do however have preferences, the most heinous acts like abuse and rape should be left 'out of the camera', it is ok to write the consequences of such things, and to shed light onto the terrible ways in which it affects a person... But to glorify it? never!
Violence is another topic I'm generally batting an eye for, I'm genuinelly more concerned with violence than sex when it comes to mature content, I can see violence in stuff like Invincible or Akira, but only if it adds to the experience... Violence for the sake of violence is pointless, and I tend to hate super gory stories.
There is also the issue of tone, the story has to be setup to allow the discussion of heavy tones beforehand, everyone is tired of the 'loss' meme, but it was a genuine shocking thing to see in comic that was basically a 'gag-a-day' strip to suddenly go super dark and touch on such a heavy subject... And a writer has to know to avoid that, not have a sudden breakdown and decide your cutesy fluflly story is going to go all 'happy tree friends' without seeting forth precedence.
So yeah, tag the story properly and make sure the reader knows what to expect from it, and in my mind you could explore any subject you damn well please... I might not like that exploration, but you can...