So I'll just share my thoughts here:
First,
I support a quickban.
Reasons:
1. It has access to a 40 BP (70 after PB) SD. Even without having to waste a turn, it can deal chip damage without having to waste a turn to set-up.
2. MKang has no hard counter. A counter's definition being:
Something that can switch in easily on a Pokemon's attacks.
Some people have listed Sableye or even Ghost-types as a counter. This is incorrect. Trevenant and Aegislash get killed by Fire Punch, with the latter also being OHKO'ed by a +2 Jolly EQ iirc. Sableye doesn't like taking any of MKang's coverage moves, nor does any of the other ghosts, and being immune to Ghost-type attacks is a huge boon, especially since Sableye can only spread burns, Taunt and it's main form of damage in Night Shade is rendered useless.
3. As others have mentioned, to kill MKang, you definitely have to have something sacrificed/severely weakened.
4. It's bulk and Speed. 105/100/100 is nothing to scoff at. With some investment, it can take a few hits and hit back. Also, it has only 1 weakness, which isn't very good in the current metagame with the introduction of fairies. Also, 100 Speed brings it to the speed tier of Celebi and other 600 BST legends and outspeeds a load of stuff.
5. It centralizes the metagame. I'm going to use the Pokebank OU stats for November:
Defensive said:
| 12 | Gliscor |
| 18 | Skarmory
| 19 | Trevenant
| 26 | Landorus-Therian |
| 53 | Sableye|
| 86 | Deoxys-Defense |
Offensive said:
| 3 | Aegislash |
| 17 | Lucario |
| 31 | Breloom |
| 69 | Ditto | (lol)
| 70 | Heracross |
| 99 | Medicham
all in the top 100. and Kangaskhan is sitting pretty at the 6th position.
Lastly- This thing actually can do things before it Mega Evolves because of Scrappy. Also, the sheer number of sets it can carry- All-out Attacker, Fake Out, SubPunch, Haxxer, and more. You have to watch out for this thing.
These factors all play a part towards it being Ubers.
Also, frail pokemon all are dead once it gets it's Power-up Punch.