x1 to x16 risers can help you be more universal, since you can use any physical PCIe slot for mining, however, they are not "better" for mining performance. Additionally, using risers can give you more flexibility with card placement (and thus cooling) outside of your case.
This is 100% correct. They are not more efficient but it takes less PCIE lanes on your CPU. Most cheap cpus are limited to 16 PCIE lanes so just having 1 16x GPU would max out the CPU. By using a 1x you can put up to 16 GPUs before you max out the PCIE lanes on your CPU.