My experience has been both. I remember, years ago, occasionally having the problem of going through many to find one with no bad sectors. But, a few years back, I dumped all my old floppies, and very few of them had issues.

I wonder if quality control was bad. It seems like the disks that were good 30 years ago are still good.

They didn't happen to be the cheapy-cheap cellophane wrapped disks? The ones that were like 20 bucks for 100? Or good brands like Verbatim, 3M or Sony?

Me too, and I didn't need to wait 30 years for that. They were corrupted when floppies were still the default.