the whole concept of immutable is focused on stability and safety of your system – yes, it is still possible to break an immutable distro, but it’s a LOT harder and takes some actual effort – there’s also a few concepts wrapped up into the “immutable” phrasing:

• immutable filesystem – the root filesystem is set as read only, updates are queued up and applied during an upgrade (some distros require a reboot, some don’t)
  • VanillaOS keeps two copies of the root system (ABroot), upgrades the inactive copy and then swaps them out
  • NixOS has everything defined in a master config file and keeps an archive of previous generations of the config file allowing you to boot into whichever generation you want
• atomicity – updates are applied individually and checked, if the update breaks then it’s reverted to the previous working state (ie. you are never left with a borked system)
• containerized apps – user space apps isolated or sandboxed in some way like Flatpaks or Docker containers or OCI so if they break, they don’t take anything else down with them
• declarative systems – the whole system (and packages and configs) are defined (declared) in one master config file – back up that config file and if something happens to your system, you just need that one file to do a full rebuild (or make an identical copy of your system on another computer) – NixOS and GNU Guix are the two more well-known in this space
  • EDIT: minor side-effect of this is you can easily tell exactly what packages are installed on your system at any given time – no hunting through history or trying to remember what you installed last month when you were testing out video players