I’ve got a couple of mint cupronickel and one copper coin that have been in these flips for around 10 years without change. I’ve never found any issues with the many hundreds I’ve used over the years either, I personally think they’re superb.
One thing I have always been obsessive about however is cleaning the coins with pure acetone before sealing them up. For anyone who has ever run a soaked cotton bud around the outside rim of a coin to remove the gunk of a thousand careful fingers, you will already know the kind of residues you are locking away with your coins. This incidentally has always been one of my main issues with slabbing. I have broken out and ‘cleaned’ slabbed coins that have left cotton buds looking like they’ve been picked up off the floor of a garage.