First off, Bahamut being in ZUZ has nothing to do with it. We aren't asking for favorites here.
Now with that out of the way, take a look at that screen shot. Notice the pyro, what weapon is he holding? That would be the backburner. Bad choice right there.
The easiest counter to a medic chain would be one air blast pyro on the team. One single pyro stops a medic chain full force. Not once, in the entire time we chained, did any pyro airblast us. Even though, there was more then one pyro coming after us.
Another way, would be to have your own uber to prevent the charging. Harder but still pretty doable.
tl;dr Why ban people for the easiest strategy to counter?