Premiere does indeed support OpenCL but CUDA is much faster and AMD cards are not as fast when it comes to video production, motion graphics or 3D rendering. Don't believe me? Puget Systems test all of this out for a range of different software - you can read it
here but below is the important paragraph
"Does Premiere Pro run better with NVIDIA or AMD?
For Premiere Pro, we have seen consistently higher performance with an NVIDIA GeForce/Quadro card than with a comparably priced AMD Radeon/Radeon Pro card. Because of this, we highly recommend that you use an NVIDIA-based card for Premiere Pro."
Developers opt for CUDA because of the huge performance gains. That is crucial for any pro.