I installed NixOS on a new computer today and spent several hours trying to work out how to get the window manager to work. The graphics are integrated and the processor is i7-8700.
In the end what worked was to set
boot.kernelPackages = pkgs.linuxPackages_4_19; boot.kernelParams = ["i195.alpha_support=1"]; services.xserver.videoDrivers = ["intel"];
I have no idea if this is something that can be upstreamed but there we go.