I use them all the time for local places and also used them in Las Vegas last year. I bought two for Swalphin restaurants as well. I never had any problem with compromised CC. Read the fine print. You usually need to purchase a minimum amount for the certificate to apply. Adult beverages may or may not count. May only work for dinner, not lunch and in my area some are not good on weekends. As noted, one per restaurant per month. You can print and reprint if lost after purchase. If the restaurant stops participating, you can trade for an new one. Currently, they do not expire.
In my experience, 90% off occurs once or twice a year at most. Usually the discount grows as the month progresses. However, in my location, some sell out at full price. Also, some of my $25 GC are regularly $17, not $10.