Eh, they're generally safer. They do tend to increase number of accidents, but because they force you to slow down and merge, they're generally minor. More fender benders, fewer fatal wrecks, isn't that a good thing?
Not every intersection can or should be replaced with a roundabout, but its a tool that is certainly underutilized in the American road designer's tool belt
Yes, but it's like 90% at least. And for the remainder, there are other options. You can make an overpass for a really important road with right exits only. That lets a lot more traffic through, while letting you get off in the middle without being stuck on it.