He wasn't a problem in URF. Where he could have four times as many mushrooms as a standard Teemo now (twice the duration, and double the CDR).
All you have to do is let a bannered minion roll down lane and your whole team can push towers without fear of running into them.
It's basic strategy to let the minions lead and take out the mushrooms for you to follow behind. Anyone can figure it out and pull it off. It will even happen by accident most of the time.
If you buy vision too then the whole game changes.
It would actually be pretty bad for teamfighting because the mushroom DoT's don't stack. So it has anti-synergy because their main teamfighting tool doesn't even stack it's damage.
I just love playing Teemo, so I'm very disappointed I don't get to see what five look like working together.
Even if he were permabanned most of the time, at least there would be the option to sometimes get to try him out in this mode.