About 5 years ago or so I was able to collect my Dad's C64 collection from my Mom's house, buy some new cables and an official C64 monitor off of eBay, and gift him his old computer back for Christmas.
I can't speak to cassettes, we had only cartridges and floppies. My Dad was a prolific pirate, so cases and cases of floppies. I'd say roughly 3 out of 5 worked, and we were able to boot the old game up. Karateka, 4th and Inches, Hat Trick, Bubble Bobble, Impossible Mission...
I was surprised the C64 worked, honestly. It had been stored for nearly a decade in an old Barn next to decrepit plow/cattle equipment from the early 1900's, not protected from the environment at all, just an old cardboard box literally busting at the seams. At least it wasn't on the ground.
That statement feels a litle misleading. The only type of cassettes produced today is Type I.
Everything else is new old stock, where you might end up with a decades-old, chemically degraded cassette.