falchard :
DisplayPort connections tend to be a little more finicky compared to HDMI, so I wouldn't really put too much weight into it. For instance take the AMD R9 290x using DisplayPort to a 28" 4k TN panel monitor at 60hz. The unique timing of the panel is not fully supported over their displayPort and it will flicker.
I think that might have just been some preliminary less advanced 4k Panels, the Samsung ones maybe?
I used the AOC U2879vf 28" 4k Freesync monitor through my R9 290's Display Port, and had absolutely zero issues, no flashing or flickering. Note, this actually had the same panel as the Samsung too, no problems
If this is what you mean:
https://youtu.be/JfreP0AESQI?t=1m11s
or with samsung's:
https://www.youtube.com/watch?v=DAASlcrA5Xc&ab_channel=%E5%B0%9A%E6%98%8C%E5%AE%81
I've never seen that before, and I have an original stock and BIOS R9 290
I upgraded to an LG 27UD68-W 4k IPS Freesync monitor, no flickering issues with that monitor now either.