Gengar is op because of its ability coupled with the fact that it had uber special attack and speed. With it allowed to roam it can easily take down choiced scarfed pokemon who are locked on the wrong move or Physical walls who are vital to the opposing teams with ease. It is not supposed to stay in on a talonflame or a scizor, its supposed to switch.
Even so, I think we should wait till manaphy/zygrade/kyerum-b are moved to OU before we have any serious discussion. Since a lot of people think mega lucario should be uber i will list out why it should not
1. we play no team preview so lucario does not know if it should set up or not
2. mach punch
3. Virtually every common choice scarfed pokemon can 1hitKO it
4. prevalence of talonflame
5. Suffers 4 move syndrome
(Phisical) Either has to give up ice punch for crunch, which will cause it to be walled by landorus/gliscor, or vice versa, and is walled by bulky ghost pokemon such as jellicant.
Or it can give up on bullet punch and it loses its priority
(special) countered by bulky sp def pokemon such as gyradoes/zapdoes/jellicant if it has vacuum wave/nastyplot/aurapshere/flashcannon or if it has dark pulse instead of vacuum wave it loses priority making it easy pray for choice scarfed pokemon.
6. not really a good reason but it's still legal in smogon.
edit: After ton of battles... yea mega luc should be banned since not many pokemon can "tank its moves" making stall almost nonexsistant.