[Lethalman]

Things you must be afraid when using nix:

1) If you install a bad grub, you cannot rollback it using nix. That means, not a wrong grub line, but if grub itself is broken.

2) Anything that is not under nix control, the data: like desktop configurations, databases, ecc.

[davexunit]

But after the first successful install you shouldn't have to worry about GRUB anymore. I don't know what it's like on NixOS, but on GuixSD we just change the grub.cfg.

[Lethalman]

You also upgrade grub versions I hope :) Or maybe improve the grub installation scripts.