I'm wondering if the presence of Haunter is too detrimental to UU. This thing has no true switch in, thus forcing many players to run Choice Scarf Absol and Choice Band Sneasel just to be sure that it won't destroy their whole team, this in turn puts many psychic types in a bad spot due to how often pursuit gets played. The only true counter this thing has might be a specially defensive Nidoqueen with pursuit but even then Will-o-wisp is a pain to deal with and Choice Specs Psychic can 2hko. The Choice Specs set can also 2hko the most prominent special walls being Kangaskan and Cradily.
252+ SpA Choice Specs Haunter Psychic vs. 252 HP / 252+ SpD Nidoqueen: 124-148 (62.9 - 75.1%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Haunter Sludge Bomb vs. 252 HP / 252+ SpD Kangaskhan: 96-114 (45.2 - 53.7%) -- 91.4% chance to 2HKO after Leftovers recovery and poison damage
252+ SpA Choice Specs Haunter Sludge Bomb vs. 252 HP / 252+ SpD Cradily: 81-96 (41.9 - 49.7%) -- 24.6% chance to 2HKO after Leftovers recovery and poison damage
However I do have to admit that Psychic is quite a rare move to be seen on haunter as it provides little coverage alongside its STAB attacks, but do keep in mind getting a Sludge Bomb poison is not that difficult so all specially defensive walls have to watch out for this nuisance. Cradily may have some leeway as it only has a small chance to get killed but it cannot punish Haunter at all.
4 Atk Cradily Rock Slide vs. 0 HP / 0 Def Haunter: 67-79 (55.8 - 65.8%) -- guaranteed 2HKO
+1 4 Atk Cradily Rock Slide vs. 0 HP / 0 Def Haunter: 100-118 (83.3 - 98.3%) -- guaranteed 2HKO
As you can see Cradily has no way to fight back meaning it has already lost the fight. Kangaskhan and Nidoqueen have dark type moves to deal with it (Substitute+Disable could stop them outright if they don't run another attacking move to hit it) but might not even be able to since dark type coverage is a hindrance to their movesets and like I said they could just be 2hko'd outright. Even if we look into possible checks to Haunter like Absol and Sneasel they are only really coming in after something has died, risking a Sludge Bomb could be an automatic loss if it hits them on the switch. Even Scizor that resists Shadow Ball and is immune to Sludge Bomb is put under pressure by Hidden Power Fire.
The only reasons why I believe it has somehow stayed in the tier is because 50/50 predictions against normal/steel types and because of how frail it is, however it manages to remedy this in multiple ways such as Substitute, Pain Split and Will-o-wisp. I find that Haunter is too strong for the tier and I personally believe it has some similar characteristics to Gengar before it banned to Ubers.
TL;DR Ban Haunter