The Steam Deck has the particular advantage of a standard configuration that game publishers are likely to test on.
That being said, I run a "nightmare stack" of sorts on my main Linux gaming PC: Alder Lake CPU, Nvidia GPU. NixOS on root and Steam installed natively. For a while, it was Unsupported Driver city - Population: Me.
...that being said, games have "just worked" for as long as I have Vulkan drivers installed. Steam might complain about software acceleration, but Cyberpunk runs with DLSS and Raytracing like a charm. The underlying stack is incredible, and I think it proves that you can make Linux a reliable delivery platform if you put in the work.
That's just my experience, but look at the rest of the industry: Apple publishing Game Porting Toolkit based on DXVK code, Raspberry Pi GPUs playing Fallout: New Vegas, RISC-V getting Box86 support, the list goes on. Hardware diversity is great right now, the Steam Deck just pulls a lot of that technology to center-stage.
It's probably not a use case people think of when they think of NixOS, but it's popular with users and contributors nonetheless, and thus also fairly well-supported.
It's also generally a nice way to cope with proprietary NVIDIA drivers (although the new open-source ones are already available in Nixpkgs as well!).
And for games with heavy anticheat, it gets a lot harder.
I think they very clearly draw the line there, nowhere has Valve claimed responsibility for fixing other developer's issues. They ship a runtime that works very similar to Windows, they don't write patches and hack jobs for bad developers.
Studios shouldn't care, exactly like OS/2 runs Windows apps better than Windows.