It depends on the game. Unreal for example is optimized for large teams, and collaboration between different kinds of professionals (programmers, artists, level designers etc.). If you're doing a one-person project relying on procedural generation and pixel graphics, you might be more productive with a much simpler game engine with a much shorter code-build-run loop
Unity is on the same league as Unreal, with the difference to target developers that rather use C# instead of C++, other than that the complexity is the same.
This subthread was in response to skohan above who said:
> If you're doing a one-person project relying on procedural generation and pixel graphics, you might be more productive with a much simpler game engine with a much shorter code-build-run loop
So I presumed you were saying "Unity is as bad as Unreal in these regards."