Roundabouts vary a lot - for automobiles some are safe, but that also relies on motorists who know the right-of-way rules. That's not always a given. It's also not clear to me that they have bicycle safety built into them. I certainly feel very exposed on them. If motorists aren't always familiar with the rules of the road for automobiles, there's a possible double problem with cyclists.
One of my students had a serious accident in a roundabout that had an explicit bike lane built into it. This was in France. I avoid them, but that's no guarantee, as there are plenty of dangerous intersections in the Boston area.