For one, roundabouts turn what would be a T-Bone intersection into a glancing hit. I think that's the biggest safety benefit. They also improve the flow of traffic since there's no starting and stopping (think of it like a stoplight is a mutex lock and a roundabout is a spinlock).
And because they have no electronic or moving parts, the upkeep is minimal. There is some cost to a traffic light, particularly as you start adding weird phases and whatnot.
