My training in kick boxing includes getting many punches on stomach. I kind of enjoy this painful training. Apart from my master everyone told its not good for stomach. Is it going to harm some internal organs if someone really hits hard many times.
One of the eternal truths about martial arts is that you're going to get hit. And another eternal truth is that sometimes you're going to get hit hard. So it's a good thing to learn to take hits. Being struck in the belly is a good way to learn to tighten the abs and discover that they are an effective shield when they are properly developed and trained.
A friend of mine used to practice boxing about thirty years ago. They did many, many sit-ups every day. He hasn't done this more than two decades. To this day, his abs are rock-hard, and punching him in the stomach is an exercise in futility.
This having been said, if someone hits you too hard, or you mess up and somehow allow the strike to get to your organs, or if you get hit too many times and your muscles get too tired to work properly, you can really mess yourself up. Your teacher should overview it all and make sure the strikes aren't too powerful, and make sure you are receiving the strikes properly.
So - it depends. It's a good drill. It can be dangerous.