When you think about it, back pain from degenerating discs should be treated the same way hip or knee pain from arthritis is treated: joint replacement. It's a rare patient that would have hip or knee fusion for joint damage from arthritis.
Fusion stops motion whereas an implant restores motion. The problems with fusion are usually related to increased movement at the level above or below the fusion. That won't happen after disc replacement. Sometimes fusions fail because the graft doesn't take and the spine still has motion. Up to half of all patients with a fusion still have pain.
As with any implant there's always a chance for wear debris to cause problems. The implant can come loose or get infected. In time we will know better what problems to expect with artificial disc replacements. For now the results are very favorable.