Unreal engine's strength has historically been editors/tools, and being feature rich, not really the engine quality which was about average.
I'm not sure about UE4, the tools are once again impressive, I haven't read the code so I don't really know how it is.
The problem isn't whether a programmer can do as good or better, the problem is how long it will take...
AFAIK, UE4 doesn't contain any tech beyond what is already publicly available if you survey the field. (ie read all GDC/SIGGRAPH... papers, and forums.)