They are surprisingly durable. I have a tape I made in 1990, it's followed me across continents and been in cardboard boxes, basements and attics. It still plays flawlessly.

Not wanting to push my luck, I archived it. On Minidisc.

I have quite a large CD collection. Two albums from the same era are unplayable due to oxidization, same band Banco De Gaia, pressed at the same plant in the UK too.