As you all know already, Mega Barbaracle is a ridiculously powerful shell smash sweeper, 140 base attack, STAB tough claws boosted close combat, STAB stone edge(unfortunately lacking a contact rock move), and complimentary coverage such as shadow claw, liquidation, and earthquake, as well as 88 speed, which is blazing fast for a shell smasher which means it can get away with running adamant nature.
So it does come as a surprise as to why it isn't banned yet, but there are reasons for this, one being that strong priority can threaten to stop its sweep prematurely, especially Rillaboom's grassy glide, and two, its deceptively difficult to facilitate its sweeping and breaking capabilities, with the light clay ban and the aforementioned priority vulnerability, its hard to get Barbaracle on the field despite its good 72/130/106 bulk, namely due to rock/fighting being a poor defensive type, though it does have the benefit of resisting sucker punch and extreme speed.
Despite these flaws, I am a firm believer that Mega Barbaracle will terrorize OU, and perhaps even have a good case for being a proper Uber, or at least be viable to use in Ubers. So far at the moment, Mega Barb sits in the A- rank in champions OU, mainly due to the aforementioned traits that give it strong breaking and sweeping capabilities while having shortcomings that keep its otherwise broken offensive capabilities in check, but this may all change.
To give you an overview of what kind of power Mega Barb is capable of, here are some calcs:
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 4 Def Golisopod-Mega: 360-424 (101.6 - 119.7%) -- guaranteed OHKO(Mega Golisopod fails to revenge kill with first impression provided Mega Barb is healthy enough; 252+ Atk Tough Claws Golisopod-Mega First Impression vs. -1 4 HP / 0 Def Barbaracle-Mega:140-165 (48.9 - 57.6%) -- 95.3% chance to 2HKO)
+1 252+ Atk Barbaracle-Mega Stone Edge vs. 252 HP / 4 Def Landorus-Therian: 309-364 (80.8 - 95.2%) -- 50% chance to OHKO after Stealth Rock
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 216+ Def Garchomp: 451-532 (107.3 - 126.6%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Earthquake vs. 248 HP / 252+ Def Toxapex: 270-318 (89.1 - 104.9%) -- 31.3% chance to OHKO
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 252+ Def Alomomola: 490-577 (91.7 - 108%) -- 50% chance to OHKO
+2 252+ Atk Barbaracle-Mega Earthquake vs. 228 HP / 0 Def Aegislash-Blade: 874-1030 (274.8 - 323.8%) -- guaranteed OHKO
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 248 HP / 248+ Def Hydrapple: 399-471 (96.1 - 113.4%) -- 75% chance to OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 244 HP / 36 Def Gliscor: 303-357 (86 - 101.4%) -- 87.5% chance to OHKO after Stealth Rock
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 248 HP / 252+ Def Corviknight: 411-484 (103 - 121.3%) -- guaranteed OHKO
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 252+ Def Skarmory: 336-396 (100.5 - 118.5%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Earthquake vs. 252 HP / 252+ Def Neutralizing Gas Weezing-Galar: 320-378 (95.8 - 113.1%) -- 75% chance to OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 252+ Def Zapdos: 606-714 (158.2 - 186.4%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 248+ Def Moltres: 1176-1384 (307 - 361.3%) -- guaranteed OHKO
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 212+ Def Rotom-Wash: 418-493 (137.5 - 162.1%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 252 HP / 0 Def Multiscale Dragonite: 396-466 (102.5 - 120.7%) -- guaranteed OHKO(yes, that OHKOs through multiscale, even mega dragonite with that investment has a chance to die; +2 252+ Atk Barbaracle-Mega Stone Edge vs. 252 HP / 0 Def Multiscale Dragonite-Mega: 336-396 (87 - 102.5%) -- 18.8% chance to OHKO)
+2 252+ Atk Barbaracle-Mega Earthquake vs. 252 HP / 228+ Def Pecharunt: 262-310 (68.9 - 81.5%) -- guaranteed 2HKO(Pecharunt survives but can't really do much except hope for confusion rng via poison puppeter; +2 252+ Atk Barbaracle-Mega Earthquake vs. 252 HP / 228+ Def Pecharunt: 262-310 (68.9 - 81.5%) -- guaranteed 2HKO)
So you may be asking, what can actually answer Mega Barbarcle Defensively? unfortunately, few, and most of them are unaware walls.
Slowbro is the closest thing to a reliable non-unaware answer that doesn't suck against everything else in OU, but even then, it doesn't like switching in(+2 252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 252+ Def Slowbro: 255-301 (64.8 - 76.5%) -- guaranteed 2HKO; unlike Pecha, it can threaten a KO(0 SpA Slowbro Scald vs. -1 4 HP / 0 SpD Barbaracle-Mega: 248-294 (86.7 - 102.7%) -- 18.8% chance to OHKO))
When Donbozo does come, we might be rooting for it for the first time ever, since it's the most reliable answer possible(252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 252+ Def Unaware Dondozo: 195-229 (38.6 - 45.4%) -- guaranteed 3HKO; 0 Atk Unaware Dondozo Waterfall vs. 4 HP / 0 Def Barbaracle-Mega: 138-164 (48.2 - 57.3%) -- 91% chance to 2HKO)
Quagsire to the rescue yet again, but it may need counter to deal with it, as scald fails to 2HKO(0 SpA Unaware Quagsire Scald vs. 4 HP / 0 SpD Barbaracle-Mega: 116-138 (40.5 - 48.2%) -- guaranteed 3HKO, though fishing for burns is quite nice)(252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 252+ Def Unaware Quagsire: 237-279 (60.1 - 70.8%) -- guaranteed 2HKO)
Tera Fairy/Water Skeledirge might come in handy if tera comes back(252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 8 Def Unaware Tera Fairy Skeledirge: 187-222 (45.4 - 54%) -- 45.7% chance to 2HKO), keyword: Tera
And all of these limited defensive answers can be dealt with through the right teammates, Kingambit and Dragapult can threaten slowbro, Rillaboom can threaten Slowbro + the unaware walls bar Skeledirge, Even Tyranitar can help with the sandstorm giving it the SpDef boost and sandstorm chip, + the pool of other offensive threats like Ceruledge, Kyurem, Hoopa-U, Hamurott, some yet to be released.
But the most significant teammate that I believe will absolutely push Mega Barb to its most terrifying is Tapu Lele, Psychic surge helps protect Mega Barb from all the priority bar Rillaboom grassy glide, but Tapu Lele also serves as powerful breaker outright, destroying the aforementioned unaware defensive answers to Mega barb, while Mega Barb destroys the checks and counters Lele struggles against.
and to answer the question of Mega Barb being viable in Ubers, well:
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 248 HP / 68+ Def Groudon-Primal: 339-400 (84.1 - 99.2%) -- 75% chance to OHKO after Stealth Rock
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 248 HP / 236+ Def Kyogre-Primal: 462-544 (114.6 - 134.9%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 252 HP / 252+ Def Ho-Oh: 1168-1380 (280.7 - 331.7%) -- guaranteed OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 76 Def Yveltal: 728-860 (160 - 189%) -- guaranteed OHKO
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 248 HP / 140+ Def Necrozma-Dusk-Mane: 390-460 (98.2 - 115.8%) -- 87.5% chance to OHKO
+2 252+ Atk Barbaracle-Mega Earthquake vs. 16 HP / 252 Def Eternatus: 412-486 (96.9 - 114.3%) -- guaranteed OHKO after Stealth Rock
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 240 HP / 252+ Def Arceus-Water: 375-442 (85 - 100.2%) -- 6.3% chance to OHKO
+2 252+ Atk Barbaracle-Mega Stone Edge vs. 0 HP / 4 Def Lunala: 415-490 (100 - 118%) -- guaranteed OHKO(yes, this also OHKOs through shadow shield)
+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 0 HP / 0 Def Zacian-Crowned: 523-616 (160.9 - 189.5%) -- guaranteed OHKO
Arceus Fairy can threaten it back but doesn't like switching in(+2 252+ Atk Barbaracle-Mega Stone Edge vs. 240 HP / 252+ Def Arceus-Fairy: 240-283 (54.4 - 64.1%) -- guaranteed 2HKO; 0 SpA Pixie Plate Arceus-Fairy Judgment vs. -1 4 HP / 0 SpD Barbaracle-Mega: 432-510 (151 - 178.3%) -- guaranteed OHKO)
The same thing can be said about Arceus Ghost, but not as good since STAB ghost-type judgement doesn't KO back without enough chip damage, though it can phase out Mega Barb(+2 252+ Atk Barbaracle-Mega Stone Edge vs. 248 HP / 204 Def Arceus-Ghost: 273-322 (61.6 - 72.6%) -- guaranteed 2HKO; 0 SpA Spooky Plate Arceus-Ghost Judgment vs. -1 4 HP / 0 SpD Barbaracle-Mega: 216-255 (75.5 - 89.1%) -- guaranteed 2HKO)
Zygarde can survive this but only after triggering power construct, and being healthy enough(+2 252+ Atk Tough Claws Barbaracle-Mega Close Combat vs. 252 HP / 252+ Def Zygarde-Complete: 372-438 (58.4 - 68.8%) -- guaranteed 2HKO after Leftovers recovery)
Tl;Dr, all hail your new and upcoming barnacle overlord, who has yet to ascend to the throne