[Sammi]

You loose local reasoning as was already said.

In theory you have the source code and you can know everything just by reading it all and debugging it all. In practice it becomes overwhelming.

Even intelligent people can only fit a little bit of information into working memory in their heads at a time. Mere mortals have no chance. We need things to be bite size and local and simple so we can fit it in our heads and reason about it.

Global variables force you to do global reasoning, which a human mind just doesn't have the capacity to do.