There is nothing wrong with the 280 Remington, except it always stood in the shadows of the 30-06 Springfield and the 270 Winchester. The 280 was introduced in the 740 semi-auto but never took off. Later it was resurrected as the 7mm Express in the bolt actions because it can obsorb higher pressures and shoot faster. It's a cartridge that makes a comeback, however it's popularity fades away quickly, overshadowed immensely and recently by the 280 AI and the 7mm-08.
The 260 Remington, well I don't know much about the cartridge except that it too sat in the shadows of the bigger 6.5 cartridges and it's ballistic twin, the 6.5x55 Swede.