I have no idea about Mari and NUKE, I'm just a hobbyist that follows the Blender mailing lists.
I didn't mean to give you the impression that I have beef with AMD. That's old news, I got over it before AMD bought ATI. I've used NVidia exclusively for the last fifteen years and know that their cards work well for me.
I'm not a GPU programmer, but from what I can tell the story is this: Brecht (the guy that wrote Cycles) originally chose CUDA for the GPU code and wrote the kernel in a monolithic fashion. CUDA can handle that, but OpenCL apparently can't; I'm not sure if that's because of something specific to Radeon cards or not, but I do know there were issues with the Radeon drivers for a while that the AMD guys had to fix. The current work is focused on splitting the large kernel into a modular system that Radeon cards can handle. AMD is helping the Cycles team with the project.
Now, I do know (from secondhand sources) that Luxrender works fine with Radeon cards, so all this is moot if you're not going to use Cycles.
However, don't think you have to go buy a Titan to render pictures. I've got a 770 and it works fine. The only advantage a new (and expensive) card gets you is that it'll be a bit faster. It's up to you to decide if that extra speed is worth the price; for me, it wasn't. Whatever you get, VRAM is what you want to focus on. If your choice is between speed and VRAM, get the extra VRAM. I've got 3 gigs and it's been enough for everything I do.