The biggest issue is they are an infrequently used traffic control device in the US. So infrequent, I don't think they even appear in the WA drivers handbook which is kind of shocking.
As a by product, you see the craziest driver behaviour in Seattle on them:
1) No one uses their indicators/turn signals as they approach. This is a major issue because it makes the roundabout incredibly ineffective, as every driver needs to treat it effectively like a 4-way stop because you don't know the intentions of the cars around you. 2) Drivers will enter the roundabout, then stop at another entryway if a car approaches. This makes no sense, as the approaching car would need to yield or stop, so it slows the traffic down again. 3) Drivers will wait at an entrance until all other cars are clear of the roundabout before entering. Again, makes no sense - you wouldn't do that on a cross-road if two cars were both indicating right etc.
Definitely need more driver education on the humble roundabout!