I think it depends somewhat on the activity.

For example, kindapping roleplays... a situation where no doesn't mean no if you're in character. I wouldn't ever enter one seriously without a safeword backup in place.

I've also encountered those who enjoy saying no and being ignored... even if it's a relatively "light" activity. Obvioulsy safeword is their only real safetey net there.

Usually, no means no of course. And more or less, I think it does a fine job. But having said that if someone asked for a safeword setup in any situation, I would never turn them down. I think I would always offer someone I was getting into a serious setup a safeword, even if I felt it was somewhat redundant, a a simple sign of good faith.