- insufficient precision of OpenGL 32-bit floats in Earth-scale computations. If you want to render the whole Earth, you'll find that at the ground level your precision is only 16m resulting in jittering as you move. This was usually solved by "zoning" and local coordinate systems, hence it was easier to deal with a smaller set of ground "tiles" and haze out the distant ones. Having said all that, nowadays you can use vertex shaders to simulate 64-bit (or rather 56-bit) precision and get around 1cm resolution at any distance
- the need to have various LODs that include curvature of the Earth with significantly increased error tolerances, which in turn increases memory consumption. Again, nowadays should be no longer an issue given entry-level GPUs having 1GB of RAM and geometry shaders