diverging diamonds handle much larger volumes than round a bouts. plus they're typically useful for interstate highway interchanges with large collector roads, so you can't really expect the high speed highway traffic to slow down and maneuver around a circle.

large traffic flows from one direction turning left will dominate a roundabout blocking access for everyone else. Especially at rush hour, you'll get people moving bumper to bumper leaving a city blocking traffic in all other directions. I ran into this in Indiana just north of Indianapolis. They have a few of the suburbs with large amounts of roundabouts. One of them was at a large industrial complex where everyone evidently left at 5. I swear 15 minutes of cars left there turning left and stopping the rest of the traffic.