They're resilient to spam, but often impossible to recover.
I had a spare SIM card that friends and family use when visiting from abroad. It's been unused for 90 days and has been deactivated. The number is lost, and irrecoverable. A friend had created a (second) Signal account with this number and can no longer log into new devices.
As a more mundane example: If I accidentally drop my phone into a river, the SIM is gone forever, and so is that line.
Sure, you can have a contract line which allows recovery. Depending on where you live, these can be several times more expensive than a regular pre-paid line.