What parts of the expanding balloon model are not very good ogies to the workings of the universe?

Well, the balloon is 3-dimensional and the surface ("space") is 2-dimensional. While it is a good ogy, I think some people might get the idea that space is "full of something" (like how the balloon is full of air) or that you could "go inside of it". Basically I mean some people may take the ogy too far. Also, the balloon has positive curvature while current observations seem to indicate the universe probably has a negative curvature. Unfortunately there is nothing (that I know of) that has negative curvature that we could compare it with.