On the subject of official (VGC) vs smogon bans there are a few factors people don't consider
Besides excluding Pokemon you can't get ingame like Deoxys, Mew or Phione and Pokemon with insanely high stats without abilities that hinder to use those aka "Ubers" like Kyurem, Reshiram or Mewtwo, the VGCs exclude Pokemon that are transfered making some Pokemon less broken like no Wish or SM Mega Kanga, Heat Wave Zapdos (on VGC2014) or Body Slam Dragonite (who would run that anyways).
It feels like we are banning more, because pokemon get suspected in smaller groups and are allowed at the beginning unlike Game Freak which has banned Pokemon from the very start, we ban/unban Pokemon over time (example: Blaziken).
In case you didn't notice, most complains come from WiFi players who get used to those rules because of things like Battle Frontier, Battle Mansion etc which already has those rules from the very start.
I don't know if Mega Mence is balanced in VGCs or Doubles (considering that Mence is already a notable threat and STAB Hyper Voice + EQ is pretty scary), but that is a completely different Meta than Singles.
It has been mentioned multiple times, but people need to realise that Mega Mence is the description of broken.
It is way harder to check like any other pokemon that was banned, the revenge kill argument which has been brought back then does not even apply on this monster (Mamo can't OHKO it with banned Ice Shards o.o), it has one set that is so good which makes it other rarely seen sets even more threatening.
I don't have the damage calculator, but if I might recall a battle I watched a week ago, +3 4 Atk Mega Mence can 2HKO a standard 0 HP/4 Def Mega Metagross and with that great bulk, it is pretty easy to set up with Mega Mence.