I know it doesn't nean that it is broken. But their design philosophy is more biased towards the front of the car. From what I see their front wing and nose is unique comparing to what others are doing. Especially the nose, they're stuck in that area for they cannot revert to a more conventional style because then they would have to make drastic changes to the rear as well.
They are overworking the front wing and the nose, that's why we see so many iterations of serrated pieces of front body work. Their whole performance depends on the ability to sweep as much clean air as possible to the back for the difusor to work properly. Therefore the difusser is heavliy relient on the front hence it cannot be freely developed on its own.
And this is the reason why I think they've reached or are almost are close to the limit of the chasis development.
Look at their sidepods for a second. Although much tighter than the W05, they are completely the same size as the W06. But they are slimmer as they go towards the rear, which leads me to believe that they have shrunk the radiators for the price of a humongous airbox.
Besides this modification and the addage of the S-duct I don't see any difference on the W07. Bunch of serrations all over the place and modified suspension geometry.
I'm not saying it isn't fast, but comparing to what Ferrari have done.... yeah I'm not very confident they are in front anymore.