My Samsung Q6FN works pretty good with my Nvidia GTX 1080 graphics card having no issues displaying 1440p @ 120hz when gaming. The only irritant is some games have bad screen tearing... which is why I bought a AMD Vega 56 card for Freesync with this TV.
The problem is that while I was able to achieve 1440p @ 120hz on the desktop with the AMD card, in most games it kicks the TV input to 4k @ 60hz - even with both the desktop and game resolutions set to 1440p. When I put the game to Windowed Mode it will show 1440p @ 120hz, but as soon as I put the game to Fullscreen the TV will show a 4k @ 60hz input. I would assume since AMD worked with Samsung to put Freesync on the TV, that AMD would want it's products to work properly with it (at least as nicely as Nvidia).
Again, this issue is not happening with my Nvidia graphics card, which leads me to believe it is an issue with AMD drivers. Not sure what else to try. If AMD can give some guidance on this OR update drivers to work nicely with the Samsung 2018 QLED line, I would greatly appreciate it.