Almost all the other dragons are good counters for him. Most notably, all those Scarfed base 100ers and Scarf Hydreigon rip apart Haxorus. Skarmory also walls him to no end, for the lulz.
I mean, Hydreigon is already in a troll speed tier, which significantly reduces his usefulness (from extremely good to just very good). Haxorus, however, is in a troll speed tier compared to Hydreigon of all things.
If Haxorus had the base 100 speed, he'd be OU definitely. But he doesn't and that's what hurts him the most.
None of the dragons can come in on a boosted Dragon Claw.
Hydreigon gets also destoryed quickly with Earthquake thanks to Mold Breaker.
The Base 100 scarfers can only counter if they switch in when Haxorus Dragon Dances ,or if they are able to predict an Earthquake. Garchomp should be able to handle boosted EQ but he isn't an idea counter if he loses over half his health.
If you're using Dragon Dance, why not Salamence who hits almost as hard physically but also OHKO's Steel types with Fire Blast? Not to mention Salamence has many more resistances and Intimidate to make switching in easier.
If you're using Swords Dance, why not Garchomp who has much better speed, secondary STAB, access to fire moves as well, and better typing and defenses?
Both Dragon Dance and Swords Dance Haxorus are outclassed. Haxorus might work on multi-dragon heavy offense teams where you're using sweepers that share counters so you can hopefully weaken a counter and let your other dragon sweep. On its own there's no good reason to use him over Salamence or Garchomp. .
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Hax will be OU mark my words, it looks cool as ****, BEST type, Sky High atk w/ good spe + DD = profit.
I know people will argue Garchomp is better, and they're right. Having said that their in no reason to not use both of them Double Dragon Teams have been with 4th gen meta game through out its time. It makes Steel a real popular choice. With Ice Shard/SR dealing with the 4th gen dragons Haxsorus doesn't have to worry about either, Ice Shard isn't killing anyone unless he's on his last leg so I see a huge sweeping posabilities for mid and late game. Also Mold breaker hitting Zong and others with EQ is somthing Chomp can't boast about doing.
Well, of course, Haxorus has some advantages over the other dragons. But do they outweight the fact that he's doubly trolled in speed and has a bad movepool? Honestly, no.
Also pointing out about the whole deal with counters... Since 4th gen most of the really good Pokemon don't have hard counters anymore. Nearly all the dragons are included. Haxorus, however, does have a pretty good hard counter in Skarmory.
That alone is telling in how less appealing Haxorus is compared to the other dragons.
Keep in mind, you asked about OU. Tiers are based on usage and not on some arbitrary standing where we determine if a Pokemon is good or not. Nevertheless, Haxorus is a good Pokemon, but his brethren are used more often, and for good reason, which in turn lowers Haxorus' usage down to UU. He might be banned to BL, but that's a different discussion.