In practice, the only time I've ever seen it used is at a table of white male neckbeards waving it about like a piety display to try and lure in girls.
Yup. The main people I've seen advocate for it are white knights and troons.
Games having ratings and warnings (this character has sexual assault in their background) I think are fine. Also the option to take a time out if things are getting a bit too much, or even discussing with the GM to make a small, surface change, as
@Wallace mentions. But that's not enough for some people, and it's a classic SJWism. They want an ability to stop and control whatever is happening, but they would swear up and down that it would never be abused an no-one could ever
conceive of misusing a form of power. Especially not someone who decides to weaponise their own potential to be triggered and/or their victimhood status, because to them that simply doesn't happen.
Though to be fair the X card also impedes one of their favourite things to do, which is to get in a snit about something and then, instead of talking to anyone about it or trying to resolve the situation, they just wait until they can complain about it on Twitter, make it sound as bad as possible, then try and get people to brigade the evildoers. I mean, it may be optimistic to think people would then respond, 'Well, why didn't you use the X Card if you were uncomfortable?' but I think at least some people would have that reasonable reaction.
In the case of the sore asses, the guy claims he just forgot to make it clear they had been shitting themselves, and was taking inspiration from teen comedies with an element of gross-out, like The In-Betweeners. Not making it clear was dumb, and honestly it is kind of a dumb idea either way. But again, the upset person didn't try and bring it up with the person directly responsible, either during or after the game. And there's a very big difference in complaining about dumb stuff from a GM and trying to get them sanctioned based on one (admittedly easy) presumption about the game.
But I just get annoyed at the move to try and accommodate this mostly fictional group of people who would be totally into roleplaying and be a totally valuable part of the game if only they weren't being made so uncomfortable by all the rapes and horrific imagery. Low confidence players needing encouragement, I get, and there are piss wizard GMs. But severely non-confrontational, easily offended spergs aren't the sort to contribute anything productive to a gaming table no matter the efforts made, so much like SJWs in general I don't see the point in giving them anything when it will never, ever be good enough.