Kinda sorta not really? The NixOS config allows you to configure GRUB, too, and doing it like that is pretty safe - plus you can boot into an older iteration and fix your mistakes via the GRUB fallback prompt if need be AFAIK.

You don't configure GRUB directly in NixOS. You configure it like everything else via NixOS modules which are pretty robust (but with enough energy, you can probably create something that passes evaluation but won't actually work). Only my VPSs use GRUB, the rest lanzaboote and none of these ever broke.

So I'd say breaking your GRUB config by accident in NixOS is very unlikely.

You would have to actively work to do so. Even then, recovery is pretty easy.