As far as I understand MacOS has no official Nvidia support (=> no Cuda), which is (at least) advised if you want to use a GPU for computing. Using OpenCL instead of CUDA would require building Tenfowlow from source. The OpenCL support is not as mature as with CUDA, so I imagine you could run into unexpected (performance) problems.
On windows, you have an excelent CUDA but lackluster Swift support.
Will they add OpenCL as a "first class backend" for Tensorflow or rather expect a "first class" support of Swift on Linux and Windows? Otherwise who is going to use it?