Bigger than it taking a lot of boilerplate is the issue that there was never sufficiently widespread and bug free support for mixing CPU and GPU code in OpenCL

Bigger than it taking a lot of boilerplate is the issue that there was never sufficiently widespread and bug free support for mixing CPU and GPU code in OpenCL in the way CUDA allows. So you end up having to do a lot more work to port stuff over. When porting stuff to CUDA, you can share a lot of code. Simplifies the task so much.