Harder in what sense? There is nothing (or very little) that makes OpenCL significantly harder by nature!
OpenCL developer tools and libraries are however a disadvantage compared to NVIDIA's CUDA stack. That's partly thanks to AMD's rather poor tools (I still hope that their OSS initiative might change that). Intel's half-assed attitude towards OpenCL support didn't help either. Most importantly, NVIDIA's attitude of intentionally crippling OpenCL on their hardware by providing piss poor dev tools, only v1.2 support, no extensions that would allow making use of their hardware's features etc. has surely contributed to successfully holding back the adoption of the OpenCL standard.
I hope the community wakes up sooner rather than later.
