Well , I would agree with Galaxy hosts on this one. The only thing that makes butcher knives problematic is that you can get hurt if you fail, but assuming that you do not fail, the handles on the knives are not dangerous. That is easier than a completely round ball because of the force vectors that will act on the ball by how you will hold it.
Some thing about butcher knives is that because they are long sticks and have handles their motion is fairly predictable and in the way they move, it is easier to move to catch them.
But consider in the ball example, the resultant force from your hand can easily be broken down rather evenly between the different directions the ball is sent in. Those primary resultant directions are up, towards your other hand, and probably away from your body in either direction that is left. That last part is going to increase with every added ball and will be harder to eliminate with a completely round ball than with a stick like butcher knives. Furthermore, when it comes to catching the balls, the balls are more likely to fall in varied places, and reaching them all before they fall to the ground is going to be far more tricky than catching knives which can be more easily controlled to move in a manage way.
One of the reasons I assume that butcher knives is such a popular answer is because Butcher knives are dangerous and people would not select the butcher knives because they are dangerous. However, when you are considering how difficult it is to SUCCEED to Juggle Butcher Knives, it is often not as usually considered, I assume, the successfully grabbing the handle of a 12 set of butcher knives successfully each time is easier than catching 12 balls. I am not sure why people conclude this.
That is why I wanted to confirm what other people thought about this, because what I think is so interesting is that this is an answer where for some reason, very few people who haven't already thought about it get it wrong. It gives a little insight into how other people think about these types of things which I think is interesting.
Afterall, if you are like me, you probably have been interested in what other people are thinking and how they think it, and since this IS a web forum where 69k+ registered users can post topics and get the response from someone of what they their thoughts are on it, I would think that everyone here is at least a little interested in other people's thoughts as well.