I also live in the UK, so I'm familiar with roundabouts for the same reasons you are, but I'm glad you've given the explanation for those that are not. In your

I also live in the UK, so I'm familiar with roundabouts for the same reasons you are, but I'm glad you've given the explanation for those that are not. In your example, "an crossroad with 4 joining roads, if only two joining roads next to each other are active and of those two, the one to the right has an endless contiguous flow, then all traffic on the road to the left is blocked", this is a theoretical weakness, but in practice it rarely occurs. For starters, the chances of zero traffic from the other two roads is small, and "continuous" traffic is rarely busy to the level where cars from the road to the left are blocked completely. Furthermore, if such bottlenecks occur, they can be mitigated against by diverting traffic, through the addition of changes to surrounding roads. Roundabouts may not be something to be implemented without consideration to the surrounding road infrastructure, but they seem like they would be decent step forward for many busy intersections with traffic coming from multiple directions.