The problem with the idea of Karma, in my humble opinion, is this:
Supposedly, you’re on this cyclical journey, spiraling ever closer to the nebulous and dilutive rejoining of spirit to "Eternal force", and you’re successive lives depend upon the actions that you’ve done in your current life.
However, here’s the problem:
Joe is a jerk in his current life. He takes advantage of people, cares only for himself, and is an all around a-hole.
So in his next life, he comes back as a dog.
So, his new owners, if they’re good people, treat him well, teach him tricks, feed him, and provide a home of love.
But for the karmic scales to truly balance out, Joe needs to get reimbursed for his sins, and getting belly rubs and potty training doesn’t do it.
So, if the new owners beat JoeDog, he get’s a taste of cosmic justice. But now his new owners will come back as dogs too, because beating dogs is bad.
So, by being good to him, they enable him to be further from true enlightenment, but by giving him his comeuppence, they propel themselves further away from enlightenment.
Yes, a tad bit strawman, but the point is the same, and that is, there is a bit of a flaw in the karmic system.