For me it's clear that the concept of karma is utterly false. We all live on the same planet, Earth, and this planet is completely random. No one is provided with even the basics of food, shelter and warmth by the planet; natural disasters come and kill the most innocent and non-deserving people, including children, who also starve and freeze to death. "Karma" says that some of these individuals are the least deserving of this kind of devastation, and that Mother Nature should provide for them, but Mother Nature doesn't exist; it's literally a planet with no feelings where the "natural" phenomenona is things like tornados, earthquakes, droughts and sub-zero temperatures.
Animals understand this, they never arrange their lives around karma and expect that purity of heart will somehow will the planet into giving them the materials needed to survive. No, we're not exactly the same as animals, but when it comes to basic survival needs, we're in the same situation; there is absolutely no magical force, karma or otherwise, that will change reality. An earthquake is not terrible in the planet's opinion, because the planet has no opinion or feelings at all, and it's going to take its course and kill the most kind-hearted and innocent of people. There is no karma-shield there.
It's the same when it comes to human actions. The planet doesn't suddenly muster up a magical force to stop those hideous actions either, nor to mysteriously get revenge on them. I understand why karma is a very attractive idea, because it *should* be true; people who hurt others should pay for their actions and face justice, and people who work hard to treat others well shouldn't be put through terrible suffering. But to me it is very clear, if we're just looking at the facts as they are, that this is something we wish was true, not the reality that we live in.
There are Buddhist monks running orphanages in Asia who claim that the disabled toddlers they're in charge of deserve their fate because they accumulated an incredible amount of bad karma over their past lives. I was sexually abused as a child, so the concept of karma really boils my piss because I did not deserve to be raped, & everyone who explicitly states or even implies that anyone innocent is responsible for their suffering is a cruel asshole.
This is one of the biggest things that makes the concept of karma so clearly false to me as well; I have heard all the platitudes of "what goes around comes around" as justification for my childhood abuse, as if any world that involved karma would allow that to happen in the first place, and now I was supposed to believe in a magical force that would vaguely get some unknown kind of justice. No, there is absolutely no way that a child deserves to go through anything remotely like that, point blank. I am really sorry you had to live through that too as it should never, never happen. This is where the belief in karma gets extremely damaging when so many people are leaning on false ideals and claiming that the force of karma will take care of us all, instead of taking real-life action to protect children.