I found that comment bizarre, someone correct me if I'm wrong but as far as I've seen in my own experience and other people in my field (3D CG) multi-GPUs seem to scale ridiculously well.
Seemed more of a case that no one wants to actually code for OpenCL on AMD...
Depends on the tools. RedShift for instance told us right after 10xx Nvidia cards came out that if we have lots of vertices (> 1 or 10 million - I don't remember exactly), you benefit from more RAM so get a Titan X. If you want to render anything with less, get more GPUs since you can't share RAM anyways, but the number of cores will help you.
Seemed more of a case that no one wants to actually code for OpenCL on AMD...