In NixOS, you have a bootloader to load your OS. Unless you botch your bootloader, you can't paint yourself into an unbootable state. If one system configuration doesn't work, you reboot and choose the prior one before the OS begins to load in a menu displayed by the bootloader.
This is also true of most regular Linux setups. Except that in those, you can only choose the kernel. Hence, if you have broken other parts of your configuration, your system might not be bootable. So the safety net is much thinner.
