Who is ranting? I think the X-card makes an excellent signal. As soon as it appears on the table, I'm out. Either the game is going to be about something potentially nasty (and thus not to my taste), or the GM thinks that some of the players are potentially snowflakes (also not too my tastes in a different way) or the GM is incorrect or pandering or perhaps merely being cautious. In the latter cases, I'm possibly abandoning an alright game, but life is too short. Also, there is a really good chance that I'm not a good fit for that table, even if the game is OK. So it is the polite thing to do to get out early.
I've had plenty of sub-par table experiences (at conventions) over the decades. These have mostly been boredom (due to an unprepared DM or a lame adventure), and occasionally a terrible DM (combination of inexperience and poor preparation). Once or twice it's been an obnoxious player (typically some kind of alpha that shouts other players down, challenges the DM all the way along, that kind of thing).
I don't think an X-card would have helped in any of those situations.
But I've got to say I'm with @jhkim here. I've been on maybe half-a-dozen tables (all but one at Gen Con) where an X-card has been on display. I don't think it guarantees anything, I don't think it restricts anything.
I've never once seen it used.
But nevertheless I do think it has value. I think it's there as a message that the DM is available and willing to take feedback, and to remind everyone that we are all there to have fun.
Sure I think players have some responsibility to not sign up for games that they might have a bad reaction to, but honestly, at Gen Con especially, it's really hard to tell beyond identifying the general genre, the paragraph descriptions can't really do the session justice.
Beyond that, I love signing up blind for stuff, trying out games and themes I have little knowledge about. I've come across some amazing gems over the years.
I think the above really only applies to conventions or pick-up store games. One of my friends (an excellent CoC Keeper) hosted a home game for a couple of new people. It ended terribly, the new players were shocked by the horror content, which was, my friend admits, maybe too graphic for a pick-up game.
Would an X-card have saved that situation? Impossible to know. But I think my friend would have liked the opportunity to be advised earlier than his guests / players were bouncing off the content much harder than he had appreciated.
But where I disagree with you is your reticence to even sit down at the table. At Gen Con at least, the X-card seems to be a room/organizer thing, or a publisher thing. It doesn't really say anything about the GM, the game / mechanics, or the other players. The only person you would be hurting is yourself.