Why ban Teemo and Karthus on One for All?
Why?
Why?
because their Ults are both particularly toxic in the One-For-All environment, so they are both disabled ever since the very first OFA.
Wasn't until after the first one that it happened.
I have a meme from a 10-man Karthus game, and a friend of mine told me that he was getting 'Nam flashbacks when he did Teemo in it.
That is when you know it is bad.
The worst part is, he wasn't even in 'Nam.
That's why i said that. XD
You just described every Mirror Mode match for Teemo... Pretty much the only time when you see all ten people taking Oracle's Extract 
5 Karthus ults who all build full AP is unplayable against. Pretty much doubles for Teemo.
why not?
So..if 5 karthus's ult..all full AP...how survivable is an ace? its forcing your entire team to buy zhonyas
That may work on one or two... Not all five if they're staggered.
It depends on what champion you're playing. Basically, 5 Karthus is a one trick pony, so if you just build enough MR to survive the ults, you win the game. Or you just build Zhonya's. Or you just happen to have picked one of the 10 or so champs who have an innate ability to dodge it or have a revive passive, and team Karthus ff's at 20.
etc.
Also, it's not enough to get one 5 man ult off. You have to keep them down by far enough that it keeps working over and over again.
Ban Karthus. Buy MR. There is no excuse to ban those champions and leave Zed, Riven, Malzahar, Ziggs. Did you play against 5 Ziggs? It is a fun mode, why everything have to be ballanced? Relax a bit.
Use your imagination. It will come to you.
Most of them wouldn't help enough if ults are staggered properly. And, for example, I don't see how sion's passive helps you there (did I miss something about it?). It's certainly possible to counterpick it, but it forces you into a narrow range on champions/strats, which I don't think is fun. As a (former, still play him often) karthus main, I support banning him from URF.