Before I actually start, I'm just going to say I'm not talking about the power of SS teams. That is a whole different issue. This is solely about Hippowdon and its effectiveness as a physical wall on a sandstorm team.
It's kind of pathetic that we're talking about banning Hippowdon after not banning Alakazam, and just so this isn't irrelevant complaining, I want to point out that we have a precedent to follow. Where as Alakazam's weakness was that he doesn't 1-2HKO "everything", Hippowdon can't even survive two of most of the attack's he's "supposed to". Offensive vs Defensive is hard to compare, but I can't be the only one that sees how much more difficult to deal with Alakazam really is. Of course, I can still argue its UU for non-precedent related reasons at the risk of being too subjective.
Let's start, as we have to, with the argument to actually ban it. "It walls too much". If this were true, I'd support banning it as it's a valid reason, but I don't believe it's actually anywhere close to being true. Well, I guess there are those who will constantly remind everyone that CBCross, CBRhyperior, and CBAggron "only" get 43% on their respective attacks, which means that Hippowdon avoids a 2HKO....without entry hazards.
There is a common misconception that Spikes and Stealth Rock are not only uncommon, but bad, never used, etc on other teams / specifically against Sandstorm teams....that's one of the most absurd things I've heard in a while. Let's think of the common Sandstorm team, then think of the common Spikers.
Hippowdon / Stoutland / Roserade / Water / Filler / Filler...etc.
Froslass and Roserade are the two most common Spikers in my experience, and Froslass sets up 2 layers on 4 of those Pokemon, 3 safely on 2 of them, and it threatens 3 of them with setting up a layer and KOing afterwards. Roserade sets up on Hippowdon (though it needs to be switched into Stealth Rock or on a double switch etc) and the ever-present bulky waters.
Rapid Spinners do exist. For example, Hitmontop, Blastoise, Donphan. How does one plan on keeping Spikes off the field when one can't switch into any Spiker? Are you going to spin on Froslass? Blastoise vs Froslass ends with 3 layers of Spikes and a Blastoise at ~35% from Pain Split and a weak Ice Beam. Hitmontop can't even touch Froslass. Donphan is not only KOed by Ice Beam flat out, but also loses if Froslass switches into anything besides Head Smash. VS Roserade? Blastoise dies. Donphan dies. Hitmontop loses, but can Spin...but those Spikes are going right back up after Hitmontop switches out. Please note for late: all of these Spinners can easily spin on Hippowdon himself and other common Sandstorm Pokemon.
So sorry, over-optimists, to rain on your parade, but you are not going to prevent hazards from being up for at least some of the game as an effective Sandstorm team.
Now that we have established the ease in which both Spikes and Stealth Rock are put on the field, it's time to really evaluate the effectiveness of Hippowdon's physical defense. All of the sudden those 43% minimum attacks, 3HKOes, become 2HKOes just like that with one layer of Spikes and no Stealth Rock. What makes matters worse is that Stoutland is almost always paired with Hippowdon meaning Pokemon such as Aggron and Rhyperior get free switch-ins. Stealth Rock alone reduces the chance of Hippowdon surviving by about half (as far as I know).
What about those people that say, "well one side having Spikes means the other side also having Spikes"? Well not only is this not true, but it's also irrelevant. Believe it or not, Heracross and Rhyperior deal the same amount of damage with ~80-90% of their HP as they do with 100%. Hippowdon cannot take the same amount of damage with only ~80-90% HP.
You may be thinking, "what does Hippowdon even wall?", and the answer is simple: not many physical heavy hitters or special attackers. However, he walls some weaker fast sweepers like Scarf Flygon, Scarf Darmanitan, and the like. Make no mistake, this is an argument in favor of Hippowdon. He also still checks those "43%" Pokemon quite easily, though Heracross is still a hard motherfucker to deal with. That's his main use. Getting him in, setting up Sandstorm, and checking a lot of Pokemon with his moderate attack and amazing defense, he is still incredibly useful. He can turn Stoutland from shit into a killing machine, making offensive teams cry as their health dwindles, and makes stall teams cry by Roaring them around with no Lefties recovery.
This is only physical attacks so far, and at the risk of sounding heavily one-sided against Hippowdon I'm just going to point out that almost every powerful Special Attacker has a way to 1-2HKO it, and pretty easily. It only resists common Special Attacks including Sludge Bomb and Thunderbolt, but those are coming from Roserade and Zapdos, what on earth is Hippowdon going to do against those two.
In conclusion and in summary, Hippowdon is UU. He is (subjective as this may be) far less scary defensive as Alakazam is offensively, so by precedent we should not ban him if we are to really believe that Alakazam is UU. But more importantly, he simply isn't good enough. Thanks for reading this whole thing / fuck you if you didn't.