For security there are these scanners:
https://github.com/flyingcircusio/vulnix
https://github.com/andir/nix-vulnerability-scanner
I also run all services in docker and my network uses VLANs behind an OPNSense firewall. I use Wireguard as a pinch point into my network to access most services. So I'm not too worried about the security aspect.
Upgrading on Nix is pretty easy - just bump your lock file and it will get the latest packages, assuming you are on the unstable channel. But unstable does break on occasion. You an also use the latest stable release of Nix and selectively choose unstable packages, which is probably the way to go. I rarely need to fix anything - it's been pretty reliable - I feel like with each patch I make to my config (which is all checked into git), the system gets more reliable and reproducible - 2 steps forward, no steps back. It only starts eating time when I want to add or upgrade some element to the system, but I always make sure to never do any action that isn't captured in Nix config and backed up, so that I don't have to come back and figure out what exactly I did or how something works again. It's been fine. Nix has a pretty steep learning curve, but considering its power, I think it's absolutely worth it.