Justification/Niche: It's Shuckle with the ability to actually take a hit... probably a lot of them, but is easy to chip away with its inability to do anything back in return. That's all it is, pretty much. It's not going to 6-0 your team... ever. So I don't think it'll overbear.
252 Atk Pure Power Medicham-Mega High Jump Kick vs. 248 HP / 252+ Def Shuckle: 136-162 (36.4 - 43.4%) -- 98.6% chance to 3HKO after Leftovers recovery
252 Atk Analytic Choice Band Gallade Close Combat vs. 248 HP / 252+ Def Shuckle: 150-177 (40.2 - 47.4%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Choice Band Tapu Bulu Wood Hammer vs. 248 HP / 252+ Def Shuckle in Grassy Terrain: 187-222 (50.1 - 59.5%) -- 25.8% chance to 2HKO after Leftovers recovery and Grassy Terrain recovery
+1 252 Atk Tough Claws Charizard-Mega-X Flare Blitz vs. 248 HP / 252+ Def Shuckle: 147-174 (39.4 - 46.6%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Swampert-Mega Waterfall vs. 248 HP / 252+ Def Shuckle in Rain: 186-218 (49.8 - 58.4%) -- guaranteed 3HKO after Leftovers recovery and Leech Seed recovery
252 Atk Choice Band Excadrill Iron Head vs. 248 HP / 252+ Def Shuckle: 156-186 (41.8 - 49.8%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Tyranitar Stone Edge vs. 248 HP / 252+ Def Shuckle: 144-170 (38.6 - 45.5%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Life Orb Sheer Force Accelgor Bug Buzz vs. 248 HP / 252+ SpD Shuckle: 99-117 (26.5 - 31.3%) -- 11.5% chance to 4HKO after Leftovers recovery
252 SpA Choice Specs Mega Launcher Absol Dark Pulse vs. 248 HP / 252+ SpD Shuckle: 114-135 (30.5 - 36.1%) -- guaranteed 4HKO after Leftovers recovery
252+ SpA Solar Power Drampa Draco Meteor vs. 248 HP / 252+ SpD Shuckle in Sun: 138-163 (36.9 - 43.6%) -- 99.5% chance to 3HKO after Leftovers recovery
252 SpA Choice Specs Froslass Blizzard vs. 248 HP / 252+ SpD Shuckle: 111-132 (29.7 - 35.3%) -- guaranteed 4HKO after Leftovers recovery
+2 252 SpA Guzzlord Draco Meteor vs. 248 HP / 252+ SpD Shuckle: 172-204 (46.1 - 54.6%) -- 7.4% chance to 2HKO after Leftovers recovery
252 SpA Tapu Lele Shattered Psyche (160 BP) vs. 248 HP / 252+ SpD Shuckle in Psychic Terrain: 153-180 (41 - 48.2%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Charizard-Mega-Y Fire Blast vs. 248 HP / 252+ SpD Shuckle in Sun: 121-144 (32.4 - 38.6%) -- 4% chance to 3HKO after Leftovers recovery
+2 252+ SpA Mew Genesis Supernova vs. 248 HP / 252+ SpD Shuckle: 213-252 (57.1 - 67.5%) -- guaranteed 2HKO after Leftovers recovery
You say it can't do anything back in return, while adding Leech Seed to its already great chipping moves of Infestation and Toxic. You also forgot that since it has so much damn bulk, it can continually set Sticky Web and Stealth Rock, simultaneously power creeping hyper offense and hyper stall. The fact that you need to use Substitute and/or Taunt to overcome this monster is overcentralizing. Yes, Chansey is similar, but the difference is that Chansey has a noticeable physical weaknesses, while Shuckle is literally the bulkiest mon in the game.
Zygarde-Complete: 636 HP, 375 Def, 227 SpD
((636*375/100000)+(636*227/100000))/2 = 1.914
Shuckle: 373 HP, 498 Def, 614 SpD
((373*498/100000)+(373*614/100000))/2 = 2.073
Being bulkier than the bulkiest Ubers Pokemon available sure is balanced.