Texture Memory

There is a limit, since it has to fit in hardware dedicated video ram.
The cons are that the map takes longer to load, both because the actual file being downloaded is larger and because there is more data to send to the video ram.
The way to decrease the use is to reduce the number and resolution of the textures. You can't do much about the base textures for the materials, but you can adjust the lightmap scale, since the lightmap is a texture too. Cubemaps are textures too. As for anything else, don't put every single model in the game in your map. They each have at least one texture each.