Upgrade the whole system? Dangerous. Upgraded and it kinda works, but you'd like to roll-back? Though luck.
NixOS is definitely not for everybody - perhaps Developer workstation is the most tricky to get right. But for production server use, the repeatability is great.
Nix ofc is agnostic of NixOS, is is generally my preferred way of getting packages.
I also happen to love when software makes too-wide assumptions and break. That is generally software poorly written and to avoid.
These side-effects and mutations cross-product with each other so much that computers have become failure dumpsters.