If you are not using Reckless, you are doing it wrong. If you use Reckless, you now outpower Mienshao's biggest competition, Terrakion, with High Jump Kick, as well as being more powerful than even Burned Guts Conkeldurr. Mienshao's niche lies in Knock Off and U-turn, which lets it act as a pivot and smash ghost types can would previously wall him, most notably Aegislash. It is also faster than M-Medicham, so it can outspeed threats like that M-Medicham can't. I think this is the best, and only set Mienshao should ever use:
Mienshao @ Choice Band/Scarf
Jolly/Naive Nature
4 Hp/ 252 Atk/ 252 Spe
-High Jump Kick
-Knock Off
-U-turn
-Stone Edge / HP Ice
There is literally no other move that warrants use. You risk wasting precious coverage and utility if you do so. This set could work well with Mega-Medicham, similar to the Double Bird strategy. Mienshao beats psychics that wall M-Medicham like Reuniclus, Slowbro, Mew, and Cress, ghost types like Cofagrigus, Aegislash, and Dusclops, and faster threats like Garchomp and Manaphy, and others if using Scarf, like Lati@s, +1 Volcarona, Specs Keldeo, Scarf Genesect, and Mega-Zard (both). On the other side, M-Medicham beats Poison types like Amoonguss and M-Venusaur, as well as Gliscor and Landorus-T on sets that lack HP Ice, and Salamence and Dragonite on sets that don't have Stone Edge. Also, here is a neat calc:
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 0 Def Scizor: 354-417 (102.9 - 121.2%) -- guaranteed OHKO
252 Atk Choice Band Reckless Mienshao High Jump Kick vs. 252 HP / 0 Def Scizor: 372-438 (108.1 - 127.3%) -- guaranteed OHKO
Mienshao is not weak
This is why I love this board. You find out so much.
Admittedly one of the biggest complaints about Mega Medicham is its barely sufficient speed. Mienshao being able to pass the power and speed of Mega Medicham's HJK is simply amazing. Few scouts can hit that hard and, if people haven't noticed they are some of the most threatening and useful team mates to have. Check out all these VoltTurn users in OU.
Landorus-T with Intimidate and 145 At backed STAB EQ
Reckless Staraptor
Technician Bullet Punch Scizor
Galewings Brave Bird Talonflame
Rotom-W ('nuff said)
Life Orb+Regenerator Hurricane Tornadus-T
Mega Manectric
Mienshao is right at home with these guys if you think about it. There's a common trend if you notice. They all have VoltTurn to scout switches, they all have at least one insanely damaging STAB attack, and they tend to have a debilitating ability, threaten with status or Knock Off to remove anything's item. And outside of Rotom, they're all pretty fast too or have priority.
Mienshao was never meant to sweep. With those kind of defenses its meant to act like most scouts, hit and run while avoiding damage all together. And people are right, Regenerator in fact sucks when the power Reckless offers would have often outright killed the very thing facing you eliminating the need for Regenerator all together. Because you don't intend on staying in, being walled by a few things or facing things with powerful priority is of no consequence when you're supposed to do what all scouts can and should do, switch out. And with the way Mienshao is played the enemy can almost never get in for free, either losing their item, being scouted with U-Turn, or simply taking more than 50% health if not outright dying from HJK. Few people would be prepared for this kind of power, as there would be walls are utterly destroyed if switching in. Resistance to stealth rock is icing on the cake. Played in this fashion by a skilled player Mienshao should almost always be of some use.
252 Atk Life Orb Reckless Mienshao High Jump Kick vs. 252 HP / 232+ Def Skarmory: 187-220 (55.9 - 65.8%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Life Orb Mienshao Hidden Power Ice vs. 252 HP / 0 SpD Gliscor: 276-328 (77.9 - 92.6%) -- 31.3% chance to OHKO after Stealth Rock
252 Atk Life Orb Reckless Mienshao High Jump Kick vs. 252 HP / 0 Def Rotom-W: 304-359 (100 - 118%) -- guaranteed OHKO
252 Atk Life Orb Reckless Mienshao High Jump Kick vs. 252 HP / 252+ Def Hippowdon: 208-246 (49.5 - 58.5%) -- 64.5% chance to 2HKO after Leftovers recovery
-1 252 Atk Life Orb Reckless Mienshao High Jump Kick vs. 252 HP / 0 Def Landorus-T: 117-138 (30.6 - 36.1%) -- guaranteed 4HKO after Leftovers recovery
0 SpA Life Orb Mienshao Hidden Power Ice vs. 252 HP / 4 SpD Landorus-T: 260-307 (68 - 80.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Reckless Mienshao High Jump Kick vs. 252 HP / 252+ Def Filter Mega Aggron: 187-222 (54.3 - 64.5%) -- guaranteed 2HKO
Are you not entertained? Having a wallbreaker and scout in one role is really handy, in fact, it's genius. Would be counters in Gliscor and Landorus-T are easily outsped and killed with HP Ice if switching in on HJK or Knock Off. The few true counter include Mega Venusaur and defensive Gyarados without Stone Edge. Defensive Tentacruel can be a competent check unless using Choice Band HJK.
I think Mienshao has the right tools to be very dangerous in this meta. She's certainly something I'm willing to investigate further.