::: The difference is less about the materials, more about the shape. A "rag joint" uses a disk, so that it has lots of articulation but little material to transmit the torque. A giubo is a thick doughnut, with the bolts passing through tubes, not just short holes. You can't use a rag joint for power transmission on modern cars, either in the prop shaft or the half shafts - it would wind up under the torque. Even my Fiat Panda used a giubo for the rear driveshaft, not a rag joint (mind you, Italians took to using these things first, in the early '60s, hence the name). Giubo are also installed pre-loaded, so that they never go into tension - if they do, the glued joint soon breaks. They're installed with a big band clamp around them. 
 ::: I haven't seen a rag joint in a steering linkage for years, don't think I've ever seen a rubber one here for articulation, although a few for vibration isolation. Practice these days is generally an all-metal coupling. I wouldn't be too surprised to see a rubber disk as a joint, but not a fat doughnut.