Mienshao was banned from UU due to how insane Mienshao was in the tier. People who played a tier like Gen 8 NU, RU, or UU while Mienshao was in that tier before getting banned know how dangerous Mienshao is. Having a base 105 speed stat, access to Regenerator, Knock Off, U-Turn, and coverage moves such as Stone Edge, Posion Jab, and Blaze Kick contributes to how insane the Martial Arts Pokemon is. Most Pokemon don't like switching into Knock Off due to its effect of removing items. U-Turn lets you chip opposing Pokemon while switching out. Regenerator would allow Mienshao to not have to worry about hazard damage and Life Orb recoil since Regenerator restores some HP to Mienshao.
In UU, there were other factors that contributed to Mienshao's Ban from UU.
However, Life Orb Mienshao has skyrocketed in popularity and dominance due to metagame changes such as the rise of Buzzwole, a common defensive counter, to OU. Moreover, the bans of Latias and Kommo-o were greatly in Mienshao's favor, removing an omnipresent faster threat that could KO it and some heavy competition as a dangerous Fighting-type wallbreaker.
Source
There were two different sets Mienshao would run: Life Orb and Choice Scarf. Choice Scarf Mienshao was fine for the tier, but Life Orb was the dangerous set in UU. I'll be covering the Life Orb set.
Life Orb Set
Mienshao @ Life Orb
Ability: Regenerator
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Close Combat
- Knock Off
- U-turn
- Poison Jab / Stone Edge / Blaze Kick
This set was the most dangerous Mienshao set in the tier. Regenerator is the main cause, as Regenerator lets you recover from LO recoil, Rocky Helmet and Entry Hazard chip to be able to fire off poweful Close Combats. Knock Off would cripple checks like Moltres and Salamence. U-Turn would let Mienshao activate Regenerator and chip opposing Pokemon while escaping from a Pokemon it cannot OHKO. The last move allowed Mienshao to choose what checks it wanted to beat, as long as your team had the support to make up for the coverage moves Mienshao didn't use. Poison Jab allowed Mienshao to damage Fairy-types such as Azumarill, Tapu Bulu, and Primarina. Stone Edge allowed you to defeat Moltres. Blaze Kick was used to beat Amoonguss.
TL;DR: Mienshao was banned from UU due to how dangerous it was in UU. Mienshao's base 105 speed, access to Knock Off, U-Turn, Regenerator, coverage moves such as Poison Jab, Stone Edge, and Blaze Kick in addition to Metagame changes such as Buzzwole's rise to OU as well as the banning of Latias and Kommo-o allowed it to use a Life Orb set that was a nightmare for UU to handle.
