Karma is such a diffuse concept in modern times. Some people believe in it religiously, meaning they don't know how it works. I see it differently. Not all immoral actions lead to bad personal outcomes and not all moral actions lead to good personal outcomes, so in that way karma doesn't work like an absolute law. It does work like a pretty good guideline in most circumstances though. It should be quite obvious that if you treat someone like sh!t there's a pretty good chance they'll do so in return, so the idea of karma isn't a complete myth. It's just a lot more mundane than many people make it out to be, it's not mystical, it's just common sense.
The golden rule (don't do to others what you wouldn't want them doing to you) is a good idea, but it's flawed. It appeals to empathy, if you don't like people punching you then don't do it to others. The one big flaw is when you'd want someone to do something to you, but they don't want you doing it to them. A great example is if you're extremely desperate for sex, there's little more you'd want than for a woman to just fvck you. According to the golden rule it would then be acceptable to throw yourself over one and fvck her, but that's called rape. The golden rule ignores what the other person don't want and assumes everyone would like what you'd like.
It's not just a matter of being true or not regarding concepts like these. It's a matter of trying to understand why the concept exists, what is really meant by it. Most people don't take careful philosophical consideration when analyzing it, thus these concepts are often misunderstood to mean something they do not.
@Fruitbat that's interesting, I did not know the Sanskrit meaning of it. So many Sanskrit words are misunderstood to the point they appear mystical, but they often refer to very mundane understandings already existent in our own language. I don't think karma is that badly misunderstood though, bad actions do in many cases lead to bad results, people just got to erase the word "always" when talking about these things.