The main idea about blueprints as I see it, is hooking up the actual level logic and graphical stuff, all other logic seam to be easily done from source.
Essentially there is no problem using the editor as a level editor and settings tool, settings up hooks for boring stuff like the ui etc.
I would say that any great game would require insight into both. The dosage you decide to use can vary.
Specially c++ is a speed increase, you can get access change compare stuff on a few lines, that would take 30 nodes in blueprint.
And the best part is, all that can be exported as a blueprint node, giving you the speed of a proper source implementation, but the easy hook up of a gui.