Mind giving a ELI5? I'm not too knowledge around this.
Forward rendering in general would allow for a prohibitively less expensive use of specialized shading for different materials, instead of the same broad one used for nearly all. You can see that in something like Cryengine or The Order where skin, vegetation, and even clothing have separate specialized PB shading in addition to the normal one covering non-metals and metals. Even metals require more specialized ones at times (anisotropic surfaces).
Regarding GI, having more lighting is just more expensive in normal forward rendering because the scene has to be redrawn for that lighting (as far as I understand it). That can be a great deal more expensive... or not... depending on a lot of factors. Being forward rendered does not mean you cannot have GI or cannot use PBS, in fact being forward rendered means your PBS can be more diverse.
I would hope forward + given xbox1.