AIUI, Unity has two types of systems the sparkly new stuff, which has the cutting-edge features but is unstable, and the old stuff, which had cutting-edge features 5 years ago but was abandoned while unstable, except in the last 5 years the bugs have been mostly documented and worked around. So the only way you can get new features is by adopting the shiny new thing, and the only way to get a stable engine is to go write one yourself. The latter is a terrible idea, FYI.