This metagame is not good for mienshao as flying priority is everywhere and it just crumbles to talonflame and mega pinsir. Also the current mets is a lot bulkier than bw ou so it does not fit into it. Moreover the ever present aegislash makes sure that mienshao can not just spam hi jump kick. it also struggles against azumarill, another priority abuser.
Talonflame, Mega Pinsir, and Azumarill are the 3 main priority users that wreck meinshao.
But Talonflame cannot switch on Stone Edge, and Knock Off does 59.3 - 70.1% and HJK does 59.7 - 70.8%, Mega Pinsir and Pinsir get pooped on by Stone Edge on the switch-in, and HJK does 49.2 - 58.4% on regular pinsir on the switch-in.
Honestly finding a check is relatively easy, but a counter is where it really counts, Azumarill is a counter, true.
Aegislash takes 58.6 - 69.1% from Knock off on the switch and 39.1 - 46.6% without an item, since Shadow Sneak does not KO, double Knock Off should kill, even if he goes for the Shadow Ball, it does take some mind games to win over King Shield, but that goes for Aegislash too.
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 0 Def Heatran: 510-603 (132.1 - 156.2%) -- guaranteed OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 252+ Def Eviolite Chansey: 712-842 (101.1 - 119.6%) -- guaranteed OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 252+ Def Blissey: 975-1149 (136.5 - 160.9%) -- guaranteed OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 252+ Def Rotom-W: 185-218 (60.8 - 71.7%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb burned Mienshao Knock Off (97.5 BP) vs. 252 HP / 252+ Def Rotom-W: 45-55 (14.8 - 18%) -- possible 8HKO after Leftovers recovery
252 Atk Life Orb burned Mienshao High Jump Kick vs. 252 HP / 252+ Def Rotom-W: 92-109 (30.2 - 35.8%) -- guaranteed 4HKO after Leftovers recovery
4 SpA Rotom-W Hydro Pump vs. 0 HP / 4 SpD Mienshao: 186-220 (68.6 - 81.1%) -- guaranteed 2HKO after burn damage
252 Atk Life Orb Mienshao Knock Off (97.5 BP) vs. 252 HP / 4 Def Deoxys-D: 177-211 (58.2 - 69.4%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Mienshao U-turn vs. 252 HP / 4 Def Deoxys-D: 130-153 (42.7 - 50.3%) -- 1.2% chance to 2HKO
-1 252 Atk Life Orb Mienshao High Jump Kick vs. 4 HP / 0 Def Mega Gyarados: 330-393 (99.3 - 118.3%) -- 93.8% chance to OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 232+ Def Skarmory: 153-183 (45.8 - 54.7%) -- 97.7% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Life Orb Mienshao Stone Edge vs. 252 HP / 252+ Def Mandibuzz: 192-229 (45.2 - 54%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 252+ Def Ferrothorn: 320-377 (90.9 - 107.1%) -- 43.8% chance to OHKO
252 Atk Life Orb Mienshao Knock Off (97.5 BP) vs. 252 HP / 252+ Def Slowbro: 182-216 (46.1 - 54.8%) -- 9% chance to 2HKO after Leftovers recovery
252 Atk Life Orb Mienshao U-turn vs. 252 HP / 252+ Def Slowbro: 133-156 (33.7 - 39.5%) -- 19.6% chance to 3HKO after Leftovers recovery
Mega Venusaur and Hippowdon are probably the only 2 viable walls that can completely wall this thing. Mega Venusaur is probably the only wall that can counter this thing.
Mienshao also checks a lot of pokemons that are priority-less and slower
252 Atk Life Orb Mienshao Stone Edge vs. 4 HP / 0 Def Mega Charizard Y: 676-801 (226.8 - 268.7%) -- guaranteed OHKO
252 Atk Life Orb Mienshao Stone Edge vs. 4 HP / 0 Def Mega Charizard X: 252-299 (84.5 - 100.3%) -- 6.3% chance to OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 4 HP / 0 Def Mega Gyarados: 502-593 (151.2 - 178.6%) -- guaranteed OHKO
-1 252 Atk Life Orb Mienshao High Jump Kick vs. 4 HP / 0 Def Mega Gyarados: 330-393 (99.3 - 118.3%) -- 93.8% chance to OHKO
252 Atk Life Orb Mienshao High Jump Kick vs. 4 HP / 0 Def Kyurem-B: 538-634 (137.2 - 161.7%) -- guaranteed OHKO
As you can see, Mienshao is a great wall breaker with a fantastic speed tier, Life Orb + regen is extremely good and overall Mienshao is extremely versatile.
I definitely think it's OU viable.