If you asked me if I write across genres, I'd say yes. Even though I'm not completely sure if that means a) writing a novel in one genre, then another novel in a different genre, etc. or b) writing a novel with strong elements of multiple genres within it. From what I can gather, the rest of the world is confused about this, too. Either way, yes, I write across genres.
My first novel is a zombie thriller. My second, a fantasy based in the world of Norse mythology. My third, historical fiction (and horror). I'm planning a YA series sometime in the future. Most of all, I'm seriously considering a series of linked novels that will enable me to write in whatever genre I feel like writing at any given time. Horror, western, noir, war, pirate...you name it.*
Taking all of that into consideration, writing horror rules over everything else.
When compared to other genres, horror can offer the greatest consequences for its characters. In a romance, the worst thing that can happen is heartbreak. In a mystery novel, the bad guy gets away. In a Western, the good guy loses the shootout.
In a horror story, everyone can die. If our heroes fail, the world itself can be at risk. And then their souls can be tormented for eternity. See where I'm going with this? In a well-crafted horror novel, the suffering never has to end. I'll allow the argument that a sci-fi alien invasion/war story offers this as well, but I'll also argue that alien monsters killing everyone and taking over our world puts it squarely into the realm of horror, regardless of how many different kinds of warp drives an author uses.
That's consequence. It's why I'm never as invested in a character's survival, or am devastated by a character's death, as I am when I read horror. I still haven't gotten over Nick Andros' death smack dab in the middle of The Stand. I mean, that guy was a deaf/mute, got the crap kicked out of him by a bully and his friends, survived a worldwide plague that killed 99.9% of the population, got shot at by a crazy chick, and his companion was a noble but slow oaf who couldn't read the notes that Nick would normally use to communicate. Despite all of this, Nick became a leader of the good guys. I loved Nick. He'd been through enough, and now was his time to shine. Then..."Hey, everybody, what's this in the closet? It looks like a b---." That hurt. Damn you, Stephen King.
Additionally, there are no excuses for extended periods of nothing happening in a horror novel. A horror writer can always force some action. Everyone too relaxed? Monster snatches someone through a window. Everyone working in perfect harmony? Have someone lose it, or turn into what they've been running from. Everyone celebrating killing the monster? Oh, too bad, turns out that it can only be killed by 1,000-year-old wood, not your bullets. The possibilities are endless, which is why the slow-paced horror novel shouldn't exist.
Try that in another genre.
My personal opinion is that horror allows for the most creative muscles to be flexed, as a writer has to balance unbelievable monsters/spirits/evil mimes with realistic human elements, as well. Writing horror isn't for everyone, but I would recommend that every new writer who is trying to find their voice at least try their hand at it.
*What can I say? Ambitious author is ambitious.