SSDs have a limited number of reads and writes. Once a unit of memory in the SSD reaches that limit, it is essentially burnt out and it can't be used again.

SSDs hide this burn out in two ways:

1. Have a little bit extra memory, so when a write fails it can be silently written to another unit instead, and the end user doesn't see the size of the drive go down.

2. Don't store data in the same place, spread data around evenly so the wear is spread around.

The problem is after a certain point, all the memory units are at the edge of burning out and the drive fails irrevocably.

Luckily, most of us aren't filling and emptying our entire drive over and over again (and in doing so wearing out the drive), so most of us won't run into this issue.

But using part of an SSD is really bad for the drive since RAM tends to be accessed and written to much more frequently which wears out the SSD.

We're seeing increased failure rates now in the M series macbooks since they use "unifed memory" (they put both RAM and VRAM in the same storage without increasing overall capacity) and needed to use their own SSDs as swap disks when they ran out of RAM/VRAM.