Nominating Lucario for B Rank over B-. Lucario is by no means a bad option for a late game sweeper due to the metagame's shift in mons that would check it under normal circumstances ie. SpDef Gliscor (+2 252+ Atk Life Orb Lucario Iron Tail vs. 244 HP / 0 Def Gliscor: 341-403 (96.8 - 114.4%) -- guaranteed OHKO after Stealth Rock), Mega Venusaur (+2 252+ Atk Life Orb Lucario Iron Tail vs. 232 HP / 0 Def Mega Venusaur: 347-409 (96.6 - 113.9%) -- guaranteed OHKO after Stealth Rock), Unaware Clefable (252+ Atk Life Orb Lucario Iron Tail vs. 252 HP / 252+ Def Unaware Clefable: 367-432 (93.1 - 109.6%) -- guaranteed OHKO after Stealth Rock). Both forms of priority are incredibly useful in the late game, allowing the Lucario user to clean up the remains of other frail, weakened mons on the opposition (Bullet Punch for Choice Scarf Terrakion vs balanced, Extreme Speed managing an OHKO on Talonflame while also avoiding annoying 50/50s that Bisharp faces with Sucker Punch). In comparison to Bisharp as a late game sweeper, Lucario compromises a STAB 80 BP attack for a less powerful 80 BP attack with +2 priority that ensures a KO without facing 50/50s. Of course, with metagame trends such as the increase of Sand Offense with Excadrill, Bisharp appears to be the overall better option, due to the sheer power that STAB Sucker Punch provides, in a generation that favors Dark type attacks. While being unable to perform a task other than sweeping with Swords Dance (Bisharp is also commonly used as a Pursuit trapper), Lucario brings to the table one thing that Bisharp does not: coverage.
Commonly refered to as "Four Move Slot Syndrome," Lucario can utilize its distribution of coverage moves (namely Ice Punch, Bullet Punch, and Iron Tail), to cater to the needs of its team. Ice Punch allows Lucario the ability to to damage targets such as Landorus-T, Zapdos, and Support Dragonite. Bullet Punch can deal with fast checks to Bird Spam such as Choice Scarf Tyranitar, Choice Scarf Terrakion, Mega Aerodactyl and can pick off weakened Magic Guard Clefable(+2 252+ Atk Life Orb Lucario Bullet Punch vs. 252 HP / 252+ Def Clefable: 291-346 (73.8 - 87.8%) -- guaranteed 2HKO after Leftovers recovery). Despite its poor accuracy, Iron Tail has several uses including, but not limited to: luring in Mega Venusaur, Clefable, and SpDef Gliscor, something Azumarill, Charizard X, Landorus, and Greninja all can benefit from.
tl;dr Lucario is underrated because it can take advantage of metagame changes, while performing its role as a late game sweeper with priority and coverage. It's nowhere near as good as Bisharp which has access to Pursuit, Knock Off, Sucker Punch, higher attack, and an amazing ability, but it can still provide team support via coverage moves.