Good and Bad News on Mint Sets.
The good news is my success rate on restoring salvageable coins from mint sets is up to about 90% for most dates. It's a multistep process for the harder to clean ones. By "salvageable" I mean coins that aren't dark and haven't been tarnished for a very long time. This includes almost all the well stored coins and many of the less well stored sets.
The bad news is the later dates appear to be starting to go as well and many of them are impossible to restore with my techniques. The '89 and '84 sets are the worst I've seen but this is a small sample size so might not apply across the board.
I'm beginning to believe they might all be in danger but it should be years until the '99 to date sets have trouble.