zero experience in this, so this might not be true. I read that due to the way most cities control parking Uber drivers often end up driving around waiting for the next fare.

Increasing cars on the road + pollution. Although you could argue there are technically less cars, it feels like there are more.

I don't know if its enough to offset the scenario you're familiar with but there's 1. people moving from other forms of transport to cars thanks to the convenience of uber and 2. journeys happening that otherwise would not have happened.