In London there's been a big move over the last decade to remove big urban roundabouts for precisely that reason. They make sense for vehicles, but they make mixed use areas much more dangerous. Roundabouts are slowly being converted to C shaped junctions with dedicated cycle routes and lanes in both direction. I imagine they're probably a lot less efficient for cars, but they also don't tend to kill people at the same rate.

I still think roundabouts are a great idea for more rural road junctions. They're much safer, and I suspect more efficient, than cross roads controlled by traffic lights. I was driving around Toronto last year and the amount of stopping and starting at traffic lights you have to do is infuriating.