You can still install Nix (package manager) on Atomic, on uBlue, it even comes pre-installed afaik.
And also, there’s Distrobox, which is totally enough if you prefer package managers over Flatpaks.
I personally like the “reliance” on Flatpaks. I think it reduces the fragmentation and makes it easier for devs, but that’s just my opinion. Do as you prefer.
I disagree (a bit at least).
Debian is just as prone to breaking due to the lack of fallbacks (e.g. Snapper), it just doesn’t break as often because it doesn’t change as much as Arch.
If you use a minimal/ default install, this won’t happen as easily, but as soon as you customise anything, you get problems.
Arch can be reliable too, there are many people who have had the same install for years without breaking.
I would actually recommend Fedora Atomic or other image based distros, e.g. VanillaOS.
They can be more modern, while being way more reliable thanks to atomic updates/ transactions, complete image rollbacks and the reproducibility.
They are a dream to use imo!