TL;DR: Having dark stuff in your story can be super important, but you need to do your homework and be empathetic to those facing those issues irl! Yes, my comic does have some of those subjects. There is no way to avoid offending people, you will upset someone, just stay respectful and be open-minded to learning new things.
I personally really like darker topics, fiction should be able to cover any type ground, imo. I mean, what better way to explore your own emotions than within the controlled realm of a story? In the US taboo stuff runs along the lines of being (In my experience):
- Certain mental illnesses
Death/murder and nonsensical violence in media is more accepted, sometimes not even questioned, or even glorified. But man, if you add in one of the things I listed above, people get on your ass! I can see how some of these things can be touchy for a lot of people, as sad as it is rape and abuse hit really close to home for many folks, either being friends and family members of victims or victims themselves. My work did a survey not too long ago relating to this, out of the thousands of us 9 out of 10 was, or knows someone who was a victim of abuse and/or sexual assault. While exploring such topics in fiction can be cathartic for some, not everyone is alright with reliving the events.
Mental illness has classically been villainize, either used to make a character look stupid, insane, or evil. While we as a species are getting better, mental health still isn't given the best treatment. Anymore people like sweeping it under the rug, or using it as a cheap sympathy cash-in. (like how illnesses like cancer gets used as sympathy fodder) The subject is normally considered taboo because people just don't like talking or even thinking about it, mental health is a very personal, and most don't like making themselves that vulnerable to others. Even denying possibly seriously mental instabilities themselves.
My dad is a good example of this, he shows very obvious signs of Narcissistic Personality Disorder, but refuses to acknowledge or even discuss the possibility of him having any mental problems. Not just because he's a boomer, and was raised that it's bad, but because it's extremely uncomfortable. (and, you know, the disorder)
Also mental health is still treated like a joke for a lot of people who just can't wrap their heads around it. My mom used to think you only got help if you were "crazy", as she put it. When she learned my uncle sought help for anger issues, possibly brought on by depression, she learned to take it more seriously.
Religion? Well, the US being an Theist evangelical country, there's a lot of feelings surrounding the inclusion of it in fiction. It doesn't help that works including this subject are either chick tracts or horror fictions where religious people are depicted as dumb monsters, or calculating murderers... or how people on both sides of the fence just blow this up in the worst way possible.
Race? Yeah, if you've schooled yourself on US history you can probably guess why this subject is taboo.
Suicide is similar to the mental health thing. It's an uncomfortable subject to talk about, usually leaving friends and family confused, and distressed.
Communities such as most of Tumblr, and the queer community kinda demonize the use of taboo subjects. Like, the very existence of something problematic being a work of fiction about queer characters is unspeakable! I'm kind of a douchebag contrarian, so I love tackling dark subjects not only because it's my personal preference, but because I've been told not to do so. I feel like they need to be discussed more, so I don't just have darker topics in my comic, I made them a major focal point. I don't care if people like it or not.
I will say, though. Research is soooo important if your going to use something that is so subjectively experienced, and in need to be taken seriously. My story focuses on mental health, abuse, some queerphobia, and will be touching on sexual assault later on. Before starting my comic I talked to people willing to discuss their experiences, therapists and psychologists about the medical side as well as their encounters with people suffering from the issues I'm working with (The HIPPA friendly version, of course), I watched videos, read research papers. Knowing the technical and personal side is incredible important. Getting the opposing pov, and the psychology behind that mindset is helpful too!
Even now I'm always learning new things and experiences from people. Also there is no real way to avoid causing problems, even the most respectful works covering dark topics can be taken badly or be seen as edgelord trash.