From a confirmed X1 dev on reddit

Somebody had PM'ed me earlier linking to the Ars article on how the PS4 has a big advantage over the Xbox one due to hUMA. This was apparently said by some AMD marketing manager.

I haven't heard of hUMA until today, so I went to look it up. The way I understand it is that in addition to having unified memory access (shared memory between CPU and GPU), which allows the GPU to read CPU memory, it is also a coherent cache system. To quote Ars, "CPU and GPU will always see a consistent view of data in memory. If one processor makes a change then the other processor will see that changed data, even if the old value was being cached."

I remember reading something on this when I got my first alpha kit. I pulled up a couple of our internal white papers and it's pretty clear that this was the exact implementation in the Xbox One's memory system.

Also I couldn't find any sources where Marc Diana actually said PS4 supports hUMA and Xbox One doesn't. Every media outlet reporting on this seems to source a single German article which I'm not inclined to believe.

http://www.reddit.com/r/xboxone/comments/1kv1xd/xbox_one_has_its_own_coherency_solution_similar/cbswrn3

hUMA, Cloud, GDDR5, blah. In the end, the PS4 is faster, that's the only thing we know for sure. What that means for games or anything else remains to be seen. Just wait for it.