I've used Hippowdon before, and find it hilarious how it can sometimes pull a comeback win from a 1 vs 3 situation. It's way better than Tyranitar when it comes to physical attacks, because almost every common physical attacker has something super effective on TTar. However, Tyranitar is much better special defensively, and its Rock/Dark STAB is useful against many Pokemon. Also, Hippowdon's primary form of offense is to use Earthquake, which isn't always feasable, since you can't always have a partner immune to it in play.
I think one of the big problems Hippowdon has is that even if it changes the weather from rain to sand, it's still at a terrible disadvantage against Rain teams. Thus, a team with Hippowdon will either have to face off against the Rain with no weather changer, or be essentially down a Pokemon from the start and bring Hippowdon.