I care about people's feelings. I agree that there's probably a qualitative difference between offending someone and causing them to have a seizure. But I'd still prefer not to offend them, all else being equal, and this would be true even if there was no such thing as epilepsy.

There's no hard line. We use our best judgement, based on what we know of our audience.

However, I will note one counterargument that I find compelling, which is utility monsters. If we reward people for getting upset, that gives them an incentive to get more upset. I don't know the best way to deal with this, but I suspect that "no trigger warnings" is not optimal.