I haven't seen the stats since I left CAP a few years back, but it used to be that over 99% of ELT activations in the US where false alarms, and the ELT failed to activate in over 95% of all crashes. In hundreds of ELT searches I've been involved with, we found: many hard landings, many UPS trucks, an EPIRB with batteries that had expired 20 years earlier, a fax machine, a life raft installed in a 747, a stuck FAA transmitter, the survival gear at the parachute shop at the local air force base and a pizza oven. Not once did the ELT signal lead to an aircraft in distress.
The COPAS-SARSAT satellites no longer monitor for 121.5 signals since they are so unreliable. At least with the 406 ELTs, the false alarm search is easier if the users have registered their units.