Safewords get held up as this magical talisman that will protect you from all harm, which is ridiculous and actually less safe than not having one at all.
A safeword is only as safe as the person you’re using it with. If they don’t stop when you say “Hey, my hand is going numb”, why would saying “rutabega” make a difference? There are people in the scene who will do whatever they think they can get away with, and these people do not give a shit about your safeword. There are also people who aren’t outright malicious but are kind of clueless. They’ll probably stop when you say stop, but five minutes later they’ll go back to doing what you just told them wasn’t working for you. There are also people who will tell you stupid bullshit like “A safeword is only for serious call-a-doctor medical distress”, which is complete and utter idiocy. Unless you have psychic powers, you simply cannot know whether someone really needed to use their safeword. And there are people who will pay lip-service to the idea of a safeword, but they’ll make it clear that if you’ll regret it if you ever try to use yours.
A safeword will not protect you from any of those people, but believing that it will can get you into trouble. If you believe that your safeword will save you, that everyone everywhere at every time will absolutely always listen to a safeword, you might end up taking much bigger risks than you meant to. BDSM is never risk free; the idea is to mitigate the risk or take calculated risks. That doesn’t work so well if you’re kidding yourself about how risky a given scene is.
Safewords can be risky from the top’s side, too. Assuming that a bottom is willing and able to use their safeword may make the top feel more comfortable, but it’s just not true that everyone will call “red” when they need to. It’s not uncommon for subs to try to prove they’re tough enough to take it (I believe this is more common with male subs, but feel free to correct me in the comments), or to refuse to ask to end the scene because they don’t want to disappoint their dom. It’s also quite possible for a bottom to be flying high on endorphins and simply not know how hard that whipping really is. Ending a scene while the bottom still wants more may disappoint them, but it’s a lot better than getting a call the next morning from someone who is freaking out about how bruised they are. Finally, some people get nonverbal when they get into subspace, and may not be capable of any kind of safeword or signal. To be clear, that’s neither better nor worse than being able to safeword no matter what’s going on, but it’s a good thing to tell your top ahead of time.
As a top, I firmly believe that if my bottom needs to use their safeword (outside of a scene that’s specifically negotiated to find out how much they can take), I’ve failed to read them as well as I should. Sometimes you do hit an emotional or physical landmine with no warning, but in general if you’re surprised when someone uses their safeword you either don’t know them well enough to push them that hard, or you just aren’t paying enough attention.
The reason the concept of a safeword exists in the first place was to allow people who enjoy resistance play or consensual non-consent to yell “Stop!” without worrying that their top would actually stop. Unless you’re doing a scene where “stop” means “I’m really into the role I’m playing” and “no” means “Keep going!”, then plain English will probably do you more good than a special safeword. “I think I’m losing circulation in my foot” gives the top a lot more information than “yellow” does. I’m not knocking “red” as a convenient shorthand for “something is badly wrong and I need the scene to end right now this instant”, but in general saying what you mean is clearer than using a code word.
Safewords can be useful, but they are absolutely not any sort of guarantee of safety.