Unified memory is what Apple is doing, other phones do, and many low end built in GPUs have done in PCs for ages. There is only one physical memory pool. Both the CPU and GPU can access it at full speed.

This means no copying between pools of memory. No speed penalty accessing the CPU memory from GPU or vice versa. If the GPU only needs 2 GB to draw the desktop it only uses 2 GB of the pool. Or it can use 45 GB if it needs it and the CPU doesn’t. But all memory has to be the same speed, and that ain’t cheap given how fast GPUs like things. I don’t know if expandable memory is possible, and they use the same bus do they compete for bandwidth. Seems theoretically easier to program for to me.

The opposite is what’s been common in graphics cards since the 2D era. CPU and GPU have their own memory and can talk over PCI/AGP/PCI-E. This is what I think they mean by shared memory, if it’s not what’s the point in touting unified?

In this model if the GPU uses 2 GB of its 12 GB total, the other 10 isn’t available to the OS at full speed and I’m not aware of any operating systems that would use it for programs/cache by default. If the GPU needs 45 GB… too bad. You have to page things in and out of GPU memory over the much slower system bus. Starting a game means loading assets into main memory then transferring them to the GPU (newer tech can accelerate this). But the CPU can have slower memory than the GPU saving money. Memory expansion on the CPU side easy. And the CPU saturating its memory bus has no effect on the speed of the GPU memory bus because it’s physically separate. More complicated memory model but it’s the one everyone uses used to.

Which is better is a matter of opinion and workload needs.