while literally no one went through the intersection for 10 or more seconds? Even a perfectly timed intersection can waste 10-20% of the time switching phases. That, ignoring space constraints or pedestrian safety issues, would work better as a roundabout, but that's not the only case. Four way stops almost always work better as a roundabout too.

It's true that once you get to a certain level of traffic, they don't work very well. For instance, more than 2 lanes and they become very difficult to navigate. Too much traffic, and no opening occurs for side streets.

Tractor-trailers manage to navigate roundabouts too. No big deal if they go up on the median a little bit. The medians are usually designed for that.

Also, roundabouts are very, very specific about who has the right of way. They even post the yield signs to help remind you how it works.