For its size, it's a very heavy car. Going uphill would thus use a lot of energy. Some of which may be recouped under braking going downhill. (I presume it has regenerative braking.)
But stop and go traffic is perhaps the only situation where an electric car has a real advantage: Electric motors do not need to idle.
Not an advantage. Stop and go traffic means brake lights are constantly used. Also factor in use of headlights, heater, radio and electronics monitoring devices and you have a big drain on power for very few miles of forward motion. My daughter has a hybrid Altima, and the regenerative brakes can be heard kicking in going down hills, so that restores some power, but not all that was used going uphill (basic physics). Regular hybrids are much better than the Volt (best use of gasoline-powered engines combined with electric motors).