Have any of you ever split curly wood? For firewood, for instance. The curly wood I've split always undulates on the curly surface. Take a look at this picture:
It's very clear that the grain makes a zigzag pattern. Since the bow will not follow this zigzag pattern, we can conclude the grain has to be violated. This violation may be relatively mild, because the zigzag is fairly shallow and does not go from one side to the other, across the limb. But it's still a violation. I'm sure this will weaken the wood, the question is just if the shallow violation is enough to cause any troubles. Some woods (such as hickory) will take mild grain violations better than others.
After all,
Sam Harper also used a piece of curly (red oak) wood...