It would be interesting to see full accounts and analysis of some situations where PLBs did not work, like the recent story of the hiker in the White Mts. One would not expert ACR to highlight those, however....
This account on the ACR page describes a PLB performance pretty much like the White Mtn hiker: his PLB's coordinates were off and varying during the rescue effort, making rescuers think he was moving. However the PLB's overall alerting function was enough to get rescuers on scene.
"The first reading of my beacon signal correctly identified my location to the rescuers. The second reading was notably off, leading rescuers and family members to mistakenly think I had moved to a different location away from the ridge. We think this occurred because of my elevation and the mountain formations around me... During the night the Personal Locater Beacon provided rescuers additional repeated signals indicating my correct position on the ridge so that the following morning rescuers knew my probable location to focus rescue operations."
I would submit that given the rocky, mountainous terrain and extreme cold, the PLB in the recent White Mountains case did a pretty good job.