I think what a lot of people seem to miss is that "No Limits" BDSM doesn't just. spawn out of nowhere- it's something you usually do with people you trust and have at least some history with or some lengthy conversations about kink. For example, a Dom friend of mine knows I have huge rape and somnophilia kinks, so she feels comfortable doing things to me in my sleep unprompted. She knows I won't be mad about those things later on, she knows they'll turn me on. She also knows I have a few hard limits that she won't cross, but aside from them? She's got a good sense of what things I don't like that aren't hard limits she could try to force me to do without any fear of consequences, because that's something I've established. So, while I'm not gonna use a safeword, I feel like I trust her enough not to harm me in a significant, lasting way.
Of course, this is pretty extreme, this kind of lifestyle isn't for everyone, but it works for me!