The best info I have gleaned is that SSD, Memory sticks, SD, CD, DVD are no more reliable long term than rotating disc magnetics. They all have a lifetime that gives you 5 years (usually) and after that you are gambling with the fates. As for magnetic tapes they suffer frequency response before you store them and then there is "print-through" and many good audio recordings have been lost on industry standard VHS cassettes because the binder oosed out long-term at the edges and they had to be baked, and even then it was a gamble. Decca made their own system on reel-reel and didn't suffer as badly.

Cloud services are better in theory because they understand and do maintenance, but when terrorists use the service, the CIA will sequestrate the whole server and you don't get to argue with them on your piffling little collection.

If the recordings are valuable, back-up yourself and copy regularly to alternative storage devices. If they are worth it, the hassle is too.