Well, HIP is not limited to AMD hardware, the runtime can also
run on NVidia hardware conforming to CUDA 6.0 or later - so if you program CUDA directly, you can convert your code to HIP and it will still run on NVidia hardware (though HIP is a subset of CUDA, so you will probably lose some functionality).
You will also need to port your code to
hip* versions of common math libraries (BLAS, RAND, SOLVER, SPARSE) - but these will also run on NVidia hardware by redirecting to standard cu* versions.
So at least it's more portable comparing to CUDA, which is firmly locked to NVidia's own hardware.