So we tried again. We got some great roles: Researcher, Scientist, Medic, and Driver (a new Role we're playtesting: it can move two cities with a drive/ferry action, and it can take other players along on drive/ferry actions).
We won a huge victory. Researcher and Scientist made such a great combination that the Scientist managed to find cures for both the purple and yellow diseases on a single turn, and the Medic and Driver ended up being incredible friends for driving around the world dispensing drugs from a firehose once a cure is found.
We found the fifth cure with four turns to spare and three diseases eradicated. We kept going trying for a total eradication, but were only able to eradicate one more disease; had we had one more turn, we could have eradicated the last disease as well.
This doesn't prove that the Driver is overpowered, but it certainly proves that there is mighty synergy possible between the Driver and the Medic.