Can a CG shader fail to work on hardware?

I’ve been writing some shaders and trying to find out if I need to write a backup for my CG shader in HLSL. All the references to HLSL in Unity I can find seem to indicate that CG/HLSL are interchangeable and you don’t need both, but I was under the impression that some graphics cards only support HLSL/CG was an NVIDIA-centric thing.

Right now I have a working CG shader with a GLSL backup if that fails to compile. Do I need to also create an HLSL backup?

No. Unity compiles Cg to assembly. Look at the compiled shader to see this. GLSL is there. Cg isn’t.