Tin Kicker and others,
A SATA Caravelle crashed in the sea in the Canaries and lots of pax did not survive this surviveable crash because they could not unfasten their seatbelts. Some had raw fingers from trying to open the slide through type as an unlatch type.
Since then it has been law to demonstrate the procedure, even though almost all aircraft have the unlatch type of buckle. I have had passengers in my car who could not unfasten belts - fortunately not in emergency case.
One problem is that many airlines mention "releasing" the belts in the demo, whereas the evacuation order calls for "Unfastening" or "Undoing". May seem a small point (if all pax understand English) but could cause trouble in a court of law.
Gerry Lee Lewis had the best song about this.