Currently, I have a laptop with NVIDIA graphics and a desktop PC with AMD graphics. The AMD stuff just works; out of the box (OSS, good performance, happy user). But NVIDIA either comes with nouveau (OSS, poor performance) or the nvidia proprietary binary driver which has all sorts of weird issues (e.g. always-on fans, animations running at different speeds, etc.).
Sure, this doesn't say anything about how a Mac would run with NVIDIA drivers, but it gives a hint that NVIDIA has its own weak spots. But the reason why Apple decided to skip on NVIDIA in this case, is probably rooted deeper within their strategies.
"Apple fully control drivers for Mac OS. But if Apple allows, our engineers are ready and eager to help Apple deliver great drivers for Mac OS 10.14 (Mojave)." -- https://devtalk.nvidia.com/default/topic/1042279/cuda-setup-...