Front fans can hurt depending on the chassis' design.
A real good example is the Bitfenix Enzo, which, if the user used front intake fans - since the fans couldn't 'breathe', what they'd do then is 'grab' the already warmed air inside the chassis and recirculate it over and over...
NZXT's H500 is another one. This one was designed around negative pressure(exhaust > intake), and the installation of front intakes works against that design to the point that it either changed nothing(it's possible neutral pressure was achieved), or made thermals worse.