I've just seen a camera sample video taken by Max Lee with the Samsung Galaxy S8 rear-facing camera. I am aware that this is a pre-production model, but I've noticed rolling shutter (jello effect). This was very noticeable in that particular sample video and it was quite shaky. From my understanding, the Samsung Galaxy S8 rear camera features an optical image stabilization and electronic IS, so it was kind of odd to see that poor performance. I really hate watching videos with "jello" effect, it just gives me headache and usually stop watching any video that have this weird optical defect.
What I am thinking is that maybe the electronic image stabilization doesn't work in 4K. That video above was shot at 4K resolution, so maybe it lacked EIS and this is why the image was a bit janky. So 4K videos might turn out to be not that good when shooting and walking, because the video stabilization might be turned off. I am not talking about the image quality here, just the jello effect.
This can certainly be an issue for me. I love shooting in 4K and I hopped that the OIS can do a fine job there, but obviously it doesn't. The phone that he shot with is pre-production model, but even then, I don't think there would be any significant changes made there. I don't know how it was in the S7, but I guess it suffers from the same thing. Full HD videos should look better though, but we'll know for sure when more Galaxy S8 sample videos are released