Those disjoint memory addresses can be an absolute pain to deal with, ask anyone who had to spend time dragging performance out of the PS3 despite it being fast

Those disjoint memory addresses can be an absolute pain to deal with, ask anyone who had to spend time dragging performance out of the PS3 despite it being faster on paper. UMA/USM can also bring it's own set of issues when you have pathological access patterns that collide with normal system memory utilization. For what its worth UMA/USM wasn't build to bridge a gap but rather to offer greater flexibility in resource utilization for embedded platforms, that's been moving upstream(along with tiling GPUs) over the years. With UMA you can page in other data on a use-case basis which is why they were relatively popular in phones, if you don't have a bunch of textures loaded on the GPU you can give that space back to other programs. Although come to think of it we used to stream audio data from GPU memory on certain consoles that didn't have large discrete system memory(the bus connecting System <-> GPU memory had some pretty harsh restrictions so you had to limit it to non-bursty, low throughput data which audio/music fit well into).