These are very frequently asked question and is very much on our radar. However as you mentioned, tracking additional outputs while conceptually simple actually greatly increases the memory footprint and amount of state that needs to be carried around which can be a drain on performance. Splitting reflections/glossy/indirect requires additional changes since the path tracer currently treats these in a unified way.
Another thing to keep in mind is that we want to prioritize realtime workflows in UE. Some of the benefits of light groups and AOVs are mainly in addressing notes that can be hard prior to having seen the image. We hope than in UE you have the ability to dial-in some of these decisions earlier in the process.
That being said, I’ll also mention that we have several in-house projects that plan on dog-fooding the Path Tracer to help us explore options in this area.