In my experience, the overwhelming majority of MB catalyst failures are due to the internals breaking apart. This usually results in customer complaints of knocking noises from the exhaust system, but sometimes the pieces will actually cause a clog, leading to drivability issues. It's extremely rare for a fuel mixture malfunction to cause catalyst damage for a couple of reasons. The various engine management systems that MB has used since the mid-nineties can trim the fuel mixture to compensate for minor problems. When the problem becomes more serious than it can adjust for, the CEL will come on, alerting the driver to a potential problem. Also, cylinder mis-fire detection is separated into two distinct categories. Catalyst Damaging, and Non-Catalyst Damaging. Problem cylinders can be shut down to prevent catalyst damage. Of course, if the driver ignores the CEL, or other symptoms, all bets are off.

The good news is, MB dealers have the ability to test catalytic converters to see if they're coming apart internally, before there's any outward indication of a problem. Whether or not this can prevent unnecessary O2 sensor replacement though, is another story.