If you install snaps from the Canonical snap store, and it is down, your systems keep running. Just that updates won't happen, and new installs will fail (well, maybe that will all work too if you have a well configured web cache in between)
If you don't like that, you can install snaps from a local file (in which case you need to handle updates yourself), or run your own snap store (I think some assembly still required)
Of the ones I looked at NIX looks particularly promising, having dependencies based on checksums sounds pretty cool.