The billion dollar question here is how to use the HSA feature in APU to avoid needing to split the RAM between GPU and CPU; in theory at least they both should be able to access same memory.
~2016 I bought a Lenovo ThinkPad E465 sporting an AMD Carrizo APU specifically to take advantage of HSA. It seemed like the feature, or at least the toolchain to take advantage of it, never really materialized. I'm glad at least someone else remembers it.