Snaps seem to work well where they work, but it's limited by their systemd dependency. So they're most useful on an LTS Ubuntu or Debian stable in order to get more up-to-date version (or missing) of a package. But if you're in a more esoteric environment with a different init, Snaps do you no good - which is a pity, because that's where they would actually be the most useful to me.

Flatpak and AppImage aren't limited in this fashion, but have few packaged apps, and they're generally all the apps I already have access to.

In practice, I've found Guix, Nix, and Docker to be most useful solutions to missing/outdated apps, though these are more complicated than Snap, Flatpak, or AppImage.