All anecdotal but hey: The roundabouts I've seen fail (we have many) are those with not enough and too much obstruction. The versions with some form of gradual grace (curved edges around a center platform or lots of grass and a bump) tend to work best. One of the worst ones was a very large one with a meter high round wall as center barrier which caused lots of damage to those who made a mistake. The too small ones are just intersections with minor obstacles to navigate.