One day I'm seeing awful jelly scroll effects on the iPad mini 6. I see what all the fuss is about. It looks bad.
Months later I'm at a different store—I go up to completely different device (but same iPad mini 6) and can't reproduce the jelly effect! It looks fine. The model next to it also didn't have it.
It seems there are many variables to this (the device's display and artificial lighting) and how our brains/eyesight works. I'm no electrician but I believe theres power phasing and light-types that can cause "frame rate" effects, and so maybe that also contributes to why some people can and can't detect it. Plus our optic nerves have a frame rate so to speak, and theres a signal processing factor because our brain is taking in many signals and has to decide what it's seeing by using algorithms.
The majority of things we see similarly, but the items that we can categorize as fringe or abnormal or artifacts, the signal processing in our brain processes them very differently. Especially at night. And especially as our optical lens shape changes. At night, a 56 year old woman is going to process headlights and street lights very differently from her 5 year old grandkids, and how those two process motion will also differ.
In addition to the ability to detect, I think there's also a personal reaction element to it. I can clearly see the jelly scroll but it doesn't bother me in the least - I never noticed it until reading about people being bothered by it. It's akin to fingernails on a blackboard - it will drive some people crazy while to others it's just a sound.