flexibity of rendering process and using shaders. It does not affect the artist in anyway, only the shader programmer.

but as Nils said, it's "very hard" to implement "wihtout" losing flexibity. Crytek makes hard things real... I used cryengine3 for a week about 7-8 months ago, integrating custom shaders was dead-easy almost didnot required an extra step compared to forward rendering. It is also up to the user to use deferred shading in cryengine3.