Apple chips share the same physical memory between the GPU and the CPU. Still, they don't have USM/UVM (Unified Shared Memory/Unified Virtual Memory), that is, the GPU and the CPU can't access the same data concurrently and easily. Programs must map/unmap pages to control which device accesses it, and that's a very expensive operation.

Not much. Synchronisation of tasks is still a big overhead.

If you've got tens to hundreds of microseconds worth of workload, sure, get the GPU to do it.

But bear in mind 1000 clocks at 4GHz is 250ns, there's still a sizeable region where tight CPU/GPU integration isn't tight enough.

I think an argument could be made depending on the real world timings. How much closer in time is the Apple GPU vs one on a PCIe bus?