This is not just a Cycles issue. Proprietary software like Octane also has this issue. I refer to Octane’s website’s FAQ. Please take a look at #5 for the answer.
An excellent article on the strengths and weaknesses of both CUDA and OpenCL:
For 3D artists, I’m really sorry to say this, but there really is no question about hardware. It’s Nvidia/Intel all the way and has been that way since Octane and Cycles started, simply because the architecture of CUDA is more conducive for GPU rendering than OpenCL, which simply does not have the functionality in that area that CUDA currently has. If you take a look at that article, OpenCL has some strong points, so, depending on which software you use, you may choose that hardware option, but, when it comes to 3D graphics, especially GPU rendering, CUDA is the only viable option at this point.
On the bright side, with recent improvements to Cycles, CPU rendering has started to creep toward GPU render speeds, depending on the speed of your CPU, and there are some arguments against GPU rendering, most notably, the fact that memory issues abound, and you need to invest insane amounts of money into a GPU just to be able to use it effectively.
That being said, I’m an Nvidia/Intel guy all the way, and have been since I had serious issues with ATI/AMD two machines ago. Unless those two really start getting going again, I’m afraid I’ll never go back to them, simply because they don’t offer the same abilities in this area than Nvidia/Intel.