Also, AFAIK, part of Nvidia's SLI requirement is that the GPU receives the PCIe lanes directly from the CPU. Also pretty sure that they limit the minimum bandwidth to cards to x8, although it may be able to be circumvented by "faking" it using PLX chips.
AMD allows for x4 lanes for crossfire out of the gate, as well as routing the GPU through the chipset. That's why crossfire boards are cheaper and can be found on non-OC board SKU's like H270 and B150 (as the manufacturer doesn't have to implement expensive PCIe switchers as well as a lot of other stuff that I don't understand/am ignorant of.)