Nix is kind-of-immutable, and you can always roll back to your old build if necessary.
But Arch on the other hand is notorious to “just break” if you don’t exactly know what you’re doing. Of course it will work perfectly reliable (apart from the few paper cuts you get when using bleeding edge stuff) if you are experienced, and optimally, if you set it up with BTRFS and Snapper/ Timeshift.
But honestly, unpopular opinion, I absolutely see no reason to use Arch today. The only exception is the DIY-aspect, which I totally understand and respect. But, for every other use case, there are better options out there, may it be Tumbleweed or Nix for a rolling release, Arch in Distrobox on Silverblue, whatever. It sounds like way too much effort for what I would get. But each to their own.