My experience as an experienced user of e.g. NixOS is that if you leave an Arch system alone for a couple of months and then try to update it, something will break horribly, because something will have changed in a non-backwards-compatible way and so I have to go and try to figure out what the new configuration should be, without good error messages. At least with NixOS, it’ll probably fail during the rebuild stage, rather than once you’ve restarted some random service or decided to reboot. I don’t really want to be doing that - I don’t have a love for system administration, I want a system which gets out of my way for me to do my work.

I don’t have to be careful about opting in to beta software or anything else with NixOS. If I want to try something, I can do so, and roll right back if it doesn’t work out. The destructive nature of Arch package installs and updates is its biggest flaw.