So what if it's new? Technology shouldn't stand in one place. MS got so scared of it, that they rushed to push their NIH lock-in alternative (DX12). It demonstrates they understand the strong potential of competition here.
Drivers can be busted anywhere (including Windows). But with Vulkan they are reduced to hardware layer which is ironed out rather fast normally. Not sure what you mean about OS integration (sounds too vague). Fullscreen, input and etc. are mostly issues related to X11, which should be cleaner with Wayland usage. The later takes longer than it should to get adopted, but all major DEs are already close to it.
Sound - never had any sound problems on Linux for a while. Are you sure you aren't measuring it by experiences of some early broken Pulse releases?
> never had any sound problems on Linux for a while.
User experience is not the same as developer experience. Debian (where Pulse seems mostly standard) is not the only distro. Vulkan is expensive, time consuming and difficult to develop against - for a game that already had multiple launch delays.
Also Vulkan support doesn't fix busted drivers, wonky fullscreen, OS integration, etc.
It also doesn't fix audio, or HID support, which is both huge headaches on linux.