In my opinion, a demo, in addition to artistic expression, is a showcase of the technical skill of its creators.

Using a pre-built engine is not quite up to the standard I would normally expect to see.

However, an argument could be made that the Unreal Engine itself is a platform (like the C64, PC, Windows, etc.), and pushing that platform's boundaries is keeping with the spirit of the demoscene.

If we restrict one from using something like Unreal, do we restrict also the use of something like OpenGL or DirectX? Or Windows/Linux itself? There are a lot of layers of abstraction we can remove.