The Witcher 3 - Medium settings - 720P resolution
I couldn't see this monster before (Ekhidna), but now it is appearing with graphic glitches.

Works fine on my RX 480, must be specific to your GPU.
Thanks. They said that it will be fixed in the next release.
RADV support of GCN 1.0 HW is in poor state itself. I tried Batman: Arkham Origins on HD7750 1GB card: the performance was suboptimal, after some time I started to get missing textures and finally the game hang. Natively this game runs fine on this HW.
I didn't try Batman Arkham Origins on Linux, but I played Batman Arkham Knight, and it works without any issues.
It was Mesa 18.1.x and LLVM 6.0 at my side so maybe it's OK nowadays.
When I use Wine without DXVK the objects render correctly but the textures is darker than it should be. The same happens with Mafia III. Could you explain that, please? @doitsujin

No, I actually can't, since both DXVK and wined3d render Witcher 3 correctly on my machine.
I guess my graphic card doesn't work well with Vulkan, and needs alot of improvements that may or may not happen in the future. Too bad for me :/
@doitsujin Guess what?!
I updated the kernel to v. 4.19 and The Witcher 3 is working perfectly now!
Manjaro uses kernel v. 4.14 BTW.