Okay...... hmm.
Safety: Do you tell the computer you have heart problems so it doesn't keep upping the stakes until your heart cries uncle?
Physical Variation: I assume it'd get baselines somehow, to account for the different heart rate bell curves for athletes vs. couch potatoes and such.
Personal Variation: This one's the big one for me. To whit:
I enjoy stressful games, such as Left 4 Dead 2, in limited amounts. And I don't push myself into intense stress all that often. Now, I like the rush, so I do want to go "out of my comfort zone" a bit, but I have to wonder at what point I'd be wanting relief while it's still pushing to make me even more stressed.
To compare: I don't like "Cringe Comedy," and when I see segments in shows I otherwise like, I often pause repeatedly, go off and do something else for a bit, just to avoid having that much "cringe" all in one sitting.
Anyway, horror, thriller, and gore are a tiny niche in my viewing/playing experience. And when I choose to view/play them, then yes, I prefer them to be dramatic, but at manageable levels.
In fact, I'm currently "playing" through horror/thriller games via Let's Plays by Helloween4545, whose soothing British accent, calming tone and off-the-wall looney reactions tone the horror/stress down to where I can enjoy it. I can't be too stressed out when he throws witty insults at the baddies, goes dancing with corpses, and responds to scary situations with bizarre non-sequiturs ("Ooh! Are those boots? Nah, not my size").
So... yeah.