I'm a new member as well, but I've been following the forums and playing competitively for some time now.
I never had too much trouble with MegaGengar or MegKhan, but I could at least understand how Shadow Tag on MegaGar essentially guarantees a kill.
Here's the main weaknesses I see with Khan:
1. Obvious. If you see a Khan in team preview, you already know that's their mega and can prep for it accordingly.
2. Predictable. Nearly all Khan's run the same set:
- Return
- Power-Up Punch
- Earthquake
- Crunch or SuckerPunch
I made my current team before MegaKhan was a popular threat, and I always find a way to deal with him.
Trevenant: Resists the first 3 moves mentioned. If Khan runs Crunch, my trev can take it and burn him right after (and my trev isn't even running max def EVs). Sitrus berry will activate and boom, I still have a relatively healthy Trev vs. a crippled Khan.
Gengar: SubDisable Gar works well if Khan isn't running Crunch. Just disable their Sucker when they predicted you to attack and you're free to wreak havoc.
Mega Absol (or any strong priority poke): If Khan is weakened enough by entry hazards, any fast poke with priority (or fast poke that resist Sucker) can threaten him.
If worst comes to worst, I send out my Klefki for the priority T-wave, allowing me to finish Khan off with a slower poke that can 1HKO him.
I will agree that there aren't very many reliable counters to Khan, but there are more than plenty checks to Khan (especially with entry hazards up).
That's just my two cents. I don't think she deserves to be banned, at least not yet.
YOU'RE BEATING A DEAD HORSE AGAIN! This was discussed 4 pages ago, TREVENANT CANNOT SWITCH/CHECK MEGAKANGA. Crunch/Sucker Punch is a must for the best MegaKanga set. And last thing you said is that you need to sacrifice pokemons to get a chance to kill MegaKanga. NO WAY!