Official comment

We do not offer processing for Full 3D job of more than 5 gigapixels. If you want to process more than that you can use a downscaling setting. The Full 3D processing is designed for covering a single structure or small area. For larger ares it is more of a map than a model which is why the 2.5D is used.
The only difference between the Full3D and the normal 2.5D processing is that overhanging areas are rendered. MOST targets don't require this. For your most recent maps with minimal overlap and trees they will actually turn out better in 2.5D.
For anything you want to render in Full 3D they need to have at least 20 looks at it from different angles.