At least in Cities Skylines 1&2, where the player has a good overview of the behavior, they only work for low to medium amount of traffic. If you have one side sending in cars constantly, the others never get an opportunity to enter the roundabout, so there’s a traffic jam spanning through the whole city behind it.
I figured that it already exists, it’s not like that’s rocket science. Tesla not doing the obviously right thing is also not surprising.