My best friend is a highly experienced civil engineer, specialized in designing intersections. His take on roundabouts:

- They are adored by urban planners because they look nice and you can put a statue or fountain in the middle

- They reduce T-bone collisions

However:

- Small fender benders and sideswipes are much more frequent

- They use way more area

- They work where each direction has an approximately equal amount of traffic. However, usually, one direction (north/south, say) has a larger share of the traffic. In those cases, the bulk of traffic all needs to slow down, downshift, turn a bit in one direction and then turn a bit in the other direction for the minority of traffic that's bleeding on or off the main artery. In those cases, a traffic light is better because the timing can be adjusted to the proportion of traffic from each direction (and more flexible since it can be adjusted over time).

- Finally, they're also (mostly) worse for cyclists and pedestrians.

Essentially, they have use cases, especially in small towns, and they're pretty, but they're usually not the right engineering solution.