The Danish authorities have undertaken a large study of the realibility of RCDs in which hundreds of installations have been tested. From 2 to 10% of the units were found not to work as intended. Some passed the test button test, but failed to work when tested with proper equipment test. The failure rate was found to increase with age and brand name products were not immune to failure.
A comprehensive report in English is available at:
The main problem, I see, is not a failure rate of 10% after 10 years, because we can easily check these devices (as electricians), but what about fuses or breakers. There are so many out there, aged 30 or 40 years, but nobody can tell whether they will trip at the correct overcurrent value after such a long time.
I haven't seen to many failed RCDs so far, and most failed on the safe side - tripped and couldn't be reset any more. Only one or two refused to trip, on one even the test resistor blew up spectacularly.
Same goes for MCBs, but I did encounter one very old range (Schrack HLS, manufacturing date stamp 1960) that refused to trip as a whole. There were a few dead shorts (faulty light socket) but the MCBs (10A H curve) never tripped before the 20A gL main fuse blew.