The problem with Mandibuzz is that unlike other Flying types such as Crobat or Salamence she can't switch into CB Heracross STABs unless she hasn't taken any prior damage at all, and even then she risks that small chance of getting 2hkoed from full.
252 Atk Choice Band Heracross Close Combat vs. 248 HP / 252+ Def Mandibuzz: 199-235 (47 - 55.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Not what I would call a reliable check, let alone a counter.
You can easily run a fairy to counter Hera (it's really not that hard to counter IMO). Crobat and Mence still have to be careful of SE, so they're only checks to Hera. Hera in general is hard to switch into since it can hit hard with the appropriate move. It's more easily checked by a plethora of mons. But, if your team doesn't have a fighting resist (or reliable fighting check), then that typing will give you trouble regardless.
But, overall I think Mandibuzz will be a great mon in the meta if it drops. I think it's immunity to weather and spore moves will give it a niche that Crobat (unless you run sleep talk CB set...) and Mence can't fulfill. It's not as crippled by Knock Off (rising of Krook) as say Crobat or Mence, while it has the ability to be a dedicated Phys wall, Sp Def Wall or mixed wall. Here are some relevant calcs that are very impressive IMO;
252+ SpA Life Orb Sheer Force Nidoqueen Ice Beam vs. 248 HP / 252+ SpD Mandibuzz: 190-224 (44.9 - 52.9%) -- guaranteed 3HKO after Leftovers recovery
-->Mandi can use Knock Off as an option to weaken Queen significantly.
252+ SpA Sheer Force Nidoqueen Ice Beam vs. 248 HP / 252+ SpD Mandibuzz: 146-172 (34.5 - 40.6%) -- 0.4% chance to 2HKO after Stealth Rock and Leftovers recovery
This only does 93.5% MAX. That second calc allows Mandi can even Roost off damage and beating Queen 1v1.
+1 252 Atk Life Orb Sheer Force Feraligatr Ice Punch vs. 248 HP / 252+ Def Mandibuzz: 250-296 (59.1 - 69.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+1 0 Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Feraligatr: 201-237 (64.6 - 76.2%) -- guaranteed 2HKO
+1 252 Atk Life Orb Salamence Outrage vs. 248 HP / 252+ Def Mandibuzz: 274-324 (64.7 - 76.5%) -- 12.5% chance to OHKO after Stealth Rock
+1 0 Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Salamence: 288-340 (87 - 102.7%) -- 18.8% chance to OHKO (add in LO damage and mence is pretty much done)
252 Atk Mega Aerodactyl Stone Edge vs. 248 HP / 252+ Def Mandibuzz: 236-278 (55.7 - 65.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 Atk Mandibuzz Foul Play vs. 40 HP / 0 Def Mega Aerodactyl: 178-211 (57.2 - 67.8%) -- guaranteed 2HKO
There are few mons in UU that can take these boosted hits, even SE hits. Knock Off/Foul Play gives it great utility along with Defog. I think that alone will give it a viability ranking similar to Umbreon or higher, easily. Definitely better than Donphan IMO...