So I just started getting into UU and I have to say that it beat my expectations as to how good it would be. This tier is very balanced and just about everything can work, which is great. I find that the idea of removing the outliers and then retesting them (UU's tiering system) to be a good idea and much better than a lot of critics make it out to be. A metagame that's always balanced is really easy to get into, and contrary to what others have said, so long as the Pokemon that's being tested isn't broken, then UU doesn't really change all the drastically in the course of the very test. Now, this segues into my next point in that I'd like to talk about the current suspect.
Haxorus's power and coverage are phenomenal no doubt, and if it sets up it's the scariest shit ever, but here's were the problem with Haxorus lies imo: setting up makes it a dangerous Pokemon, but the issue is actually trying to set up and make use of the boosts for more than three turns, which almost never happens in practice. I didn't specifically prepare for Haxorus, and I found that just about everything on my team can either A) severely damage / cripple it to the point where setup is risky or B) weaken it to the point where I can revenge kill it with my Scarf / Priority user. Granted, stall teams are going to struggle with SD variants, but those aren't exactly too hot against offense due to its middling Speed, so it makes the set a bit matchup dependant, so by using the set, you're running the risk of it not being able to perform as consistently against all types of playstyles just like the rest of your team. Even if for w/e reason it was able to set up vs offense (Good luck with that!), just about every scarfer can pick it off, Mienshao's HJK, Hydreigon's Draco Meteor, etc, and there's more than enough priority to snipe Haxorus too, such as Absol's Sucker Punch, Entei's ExtremeSpeed and so on. Its Speed is probably the biggest downfall, simply because unlike Salamence, it can't outspeed one of the most important Scarf users at +1 (Hydreigon) and it doesn't have the bulk to force other common Scarf users to revenge kill it via suboptimal moves as opposed to their STABs. Between its above average-ish Speed and so-so bulk, I don't find it a problem to keep Haxorus from setting up or even stopping it when it does so.
I ran the SD set under Sticky Web and that was probably the most effective its been in my experience. I basically needed a stallbreaker on my team, and Haxorus could also take advantage of the extra Speed to not be revenge killed as easily. Even against opposing stall, setup was difficult. I had to make sure to keep Lum intact in order to comfortably set up on Blissey, and even when I did get the chance to set up, there were things that could stand in its way like Ice Beam Slowbro, Hippowdon, Mega Aggron, etc. The same problem kept on reoccurring was that it just couldn't set up all too safely. My opinion is that stall should almost always use a Fairy and a fat Steel, so between those two kinds of Pokemon and other physical walls thrown in the mix (to bait out Outrage for instance), Haxorus isn't going to be setting up without a fight, and it is possible to prevent it from getting more than one kill per game. I feel that while Haxorus is a definite threat for defensive builds, it's not impossible to deal with and there are good enough ways to adapt to it without running obscure Pokemon, but rather, simply switching out a move or two on a select few walls to better handle it.
tl;dr
Dragon Dance sets have difficulty actually setting up vs offense even then, it can't pull of a very easy sweep when setup is attainted anyway.
Haxorus is just sorta average vs offense and the SD set (the one that supposedly rips open stall) is a tad matchup dependant unless you run it with the right support, which is a bit disappointing imo.
Defensive builds can actually pressure Haxorus due to its average bulk and can bait out Outrage, thus allowing a Fairy / Steel to handle it. It isn't a hard Pokemon to play around due to the drawbacks affiliated if it uses Outrage or the significantly lower damage output if it goes for Dragon Claw.
All in all, Haxorus just doesn't seem broken from my perspective and fits like a glove in UU.