I do suffer from PTSD and I find trigger warnings deeply unhelpful. If anything, they raise my blood pressure and short circuit the process of reading context for potential stressors, which is IMHO very necessary for managing and eventually recovering from stress.

I see three problems with trigger warnings. One is that some people have developed a habit of writing 'trigger warning for blah' at the beginning of something and then just going on to vent or throw the literary equivalent of a pity party, which readers are implicitly forbidden to criticize in any way. Another is that the worthy purpose of warning others about the potential for offence can easily degenerate into social engineering in the hands of an unscrupulous person. One form of this is to demand special treatment, while another form is to deliberately trigger anxiety in others while evading responsibility for it by pointing to existence of the trigger warning.

The biggest problem by far though is that anything could be triggering to someone - like a perverse version of rule 34, if it exists there is a traumatic version of it. This is the entire basis of the horror movie genre: you take something that looks innocuous and make it into something scary by juxtaposing it with something awful, in order to create suspenseful dread the next time you see the innocuous thing. This is why creature features aren't really horror movies; while it's horrific to depict someone eaten by a shark or a giant bear or whatever, you already know those things are dangerous and if they show up you are in trouble. So while you might feel scared watching Jaws, you knew from the outset that you were (vicariously) going into a hazardous situation, and conversely that you can avoid scary encounters with hungry sharks by not swimming in the ocean or visiting aquariums. But if I show you a movie where, I dunno, playing certain chords on an old piano can summon an evil spirit* and the movie is convincingly scary, then you'll get a little shudder every time you see a piano lurking in the corner of the room.

* this seems like a pretty stupid premise, but in medieval times certain dissonant note combinations were avoided in western church music because the harmonic instability was considered too evil-sounding. A modern parallel is to sing a well-known children's song in a minor key, which invariably sounds creepy.