As I understand it, that has to work for the CORAL 2 US "exascale", so people who've been proved fairly right so far obviously have some confidence in it. (de Supinksi of Livermore said he'd be out of a job if conventional wisdom was right, though it was pretty obvious at the time that it wasn't.)
Free software too, praise be.
There's also HIP[1], which can be used as a thin wrapper around CUDA, or with the ROCm backend on AMD platforms. It doesn't yet match CUDA in either breadth of features or maturity, but it's getting closer every day.
[1]: https://github.com/ROCm-Developer-Tools/HIP