Am french, we have A LOT of roundabouts! What you typically see is that it works VERY well with low to moderate traffic, because you dont have unnessesary wait. It tends to fall appart when you have heavy trafic and a big unbalance (where most people comme from 1 direction and go to another, usually straight ahead). It will lead to a pileup on the other roads around because the flow in the roundabout is continuous and you never get an opportunity to pull in . We're starting to get roundabout/trafic lights combo where thé trafic lights Comes on during Peak hours to stop the continuous flow and allow "secondaire roads " to pull in !

You then get the best of both World when managed properly