While poor eyesight could potentially have some impact, I honestly don't believe that explains the differences people are seeing. I do believe that other factors, such as a person's IPD and distance from the lenses would have a greater impact overall for people using the same headset. You can even test this yourself, by moving around the headset and you'll notice the intensity of the rays can change dramatically.
I've now used 6 Rifts and no two of them performed the same. These rays are one area where there has been a difference. The Rifts which had the least impactful rays were those which appeared to have issues with their displays, with all of them being noticeably dimmer and many having the red tint problem.