Indeed CUDA is nice due to the way it uses C++, integrates host and GPU code in a single file, and in the convenience of compilation. Basically I think CUDA is a bit easier to start with than OpenCL.

OTOH CUDA only works on Nvidia, and that's a major limitation.

GpuOwl uses heavily FP64 ("double" floating point), and FP64 is more readily available at consumer prices on AMD GPUs. We (the GIMPS project) use a lot of Radeon VII and Radeon Pro VII GPUs, which have great FP64 at a cheap price (I am personally running 8x Radeon Pro VII that I bought new for about $300 a piece).

So you see, for us AMD GPUs are the first citizen. Of course I want to support Nvidia GPUs as well, and OpenCL allows that. Luke Durant did run GpuOwl on a lot of Nvidia GPUs in the cloud, and I'm happy GpuOwl did work well for him on Nvidia.