Man, after that last book wrecked me for a week, I became a total convert to checking warnings. It's not about spoilers for me—it's about knowing what I can handle on a given Tuesday. I always look up 'does the dog die' type sites and skim Goodreads reviews for mentions of specific stuff.
I think the big ones are the major relationship betrayals, especially if it's framed as a romance. Non-con or dubious consent scenes are a big one a lot of readers flag. Graphic violence, particularly against kids or animals, can be a hard stop. I also check for things like miscarriage or pregnancy loss, because some books throw that in as a plot device with zero warning, and that can be a real gut punch.
For me, the emotional triggers are often in the smaller details too, like prolonged, detailed descriptions of a panic attack or a really bleak, hopeless depressive spiral. A happy ending doesn't always erase the journey there.
My library app has started adding content notes, which is a huge step forward. I wish more publishers would just put a discreet list in the front matter.