I think part of what makes DFG strong is the sort of % hp stacking meta going on right now. Resistances are invalidated because of the way penetration is calculated now, and the fact that you can stack as much hp as you like but when it's all based off of a % of that anyways, it's kinda useless. Almost all of the new champs have a % hp move, and it really shows how hard that kind of thing is to balance. A while ago, some rioter (Morello I think) made a post about how it's fine to have one really strong gimmick thing, but when it starts to become more available they all have to be toned down. His example was Amumu's ulti. If Amumu was the only one with an ulti like that, then it could be much stronger because it was the only one like it. However, because there are so many similar (Oriana, Sejuani, Galio, Malphite, Nami, Morgana, Kennen) they all have to be toned down. In fact, so many champs have a huge aoe cc ulti thing now that it's past critical mass and they can't be made any weaker, because you can only have 5 at any given time.
The same thing is happening with % HP, only they haven't been toned down. Almost all of the popular champs right now can shove % hp somewhere into their kit, either through base skills like Elise, or through items like Ahri. It's really interesting when you look at it from a design perspective.