I believe there is a hardcoded limit of 100, the reason probably being speed issues. I found this 'feature' quite annoying a while ago since it was not metioned anywhere in the help. I think the limit is around 100 but the reason is that lists are not efficient for this sort of operation. Arrays are much more efficient as the size increases. Hence for large lists it becomes a slow process.
Arrays are a true assignment to a placeholder.
I don't know if this is written down anywhere. I agree it should be in the Help pages though. You can still have much longer lists as long as you don't assign to them. I guess I'll start using arrays instead since they are faster. This is perfectly fine, as long as x is not previously defined as a list.