User blog comment:RoznosicielCiast/Is Siphoning Strike too strong?/@comment-6000413-20131105150041

They could reduce the stacking power of Nasus in late game so that you gain full first stack and every following one is reduced by (0.004*number of stacks) or (0.001*bonus damage). This way you could keep nasus early at his current stage (not so great), while nerfing his late game ability to stack power.

It's a mechanich similar to Thresh's and UI think it should be applied to all infinite stackings (with different ratios: veigar's and Sion's don't add that much power to the champs.