"The major issue i have with both of these tools is that the responsibility is on the person who is uncomfortable with the subject matter to speak out."
I guess my issue here is that I don't understand the alternative. Whether you use the X-Card or the "Be excellent to each other" boilerplate or any standard politeness metric or what-have-you, there isn't a way to access what people are uncomfortable with that isn't communication. You can ask people what they're comfortable with outright, you can encourage them to speak up if there's an issue that you're not aware of, you can pay attention and check in periodically even if the explicit tool or tools in use haven't been engaged, and so on. Some of these "back to basics" sorts of maneuvers are just less formalized versions of popular safety tools for hopefully obvious reasons. They're mostly just nice efficient packages for the same sorts of rules that keep our friends safe in any less formal conversation.
But nothing, formalized tool or not, functions when the person who is uncomfortable does not speak out. The rest of the table can use all sorts of tools (x-card, "back to basics", whatever) to try and encourage that and can hunt around for whatever best fits the group, but these are all just frameworks for communication. You can't skip the communication part and even "back to basics" doesn't skip it. You're always going to be left with guessing what someone might be comfortable with until room is created for them to speak up and they take advantage of that.
I get why it sounds right to say that it shouldn't be your responsibility to compel everyone to treat you with respect and dignity. I don't get why it sounds right to expect other people to know what treating you with respect and dignity looks like in minute detail without you stepping into that conversation. There will never be a substitute for communicating your needs and preferences. If you can't trust the people at your table to respect that or hear that, there's a problem. Maybe there's a problem with those people or with the community that those people were drawn from or with the kinds of things that particular game is encouraging people at the table to do, and so on and so forth. But I would echo the earlier sentiment that if people are coming to the table and unilaterally deciding to push up close to people's boundaries ... the biggest problem isn't naming those boundaries but those people. I can see why naming those boundaries might make it worse--there's always going to be a bit of the "don't think of the white elephant" problem even in good faith, but that is miles away from playing chicken with the comfort of other people at the table as you describe players doing in that thread.
That's not a broken safety tool. That's an asshole, and there isn't a safe way to play with people who are not interested in your comfort.