Depending on how you look at things, there is a lot of variation among coaster brakes. Of course, most of those variations will seem pretty insignificant to anyone who isn't, like, really into coaster brakes...

Some coaster brakes have internal discs, while most modern ones have internal drums. Some have roller-bearing clutches (old Sachs, Velosteel) which definitely reduce drag, but I've yet to meet a coaster brake hub that didn't hae some drag, which tends to be noticeable mostly on the stand or low speeds. (When I am walking any of my bikes with the Shimano cb-e110, I can definitely feel the drag. Once I'm over prolly about 5mph, I don't feel it anymore...but it's likely still there.) The thing is, most folks who are running a c/b hub are not so worried about performance and efficiency as they are about low maintenance and simplicity, which are areas in which the C/B hub shines.