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In practice, I've found Guix, Nix, and Docker to be most useful solutions to missing/outdated apps, though these are more complicated than Snap, Flatpak, or AppImage.
> Snaps seem to work well where they work, but it's limited by their systemd dependency.
A more serious issue with snaps is that they rely on AppArmor as a security mechanism, which is not actually present on most Linux systems (only Ubuntu variants, SUSE and Solus). The snaps will still run elsewhere, but not with the same security as you might think you were getting.
> will installing snapd not install AppArmor as well?
It can't, because AppArmor is a kernel-level feature that also requires some level of integration into the rest of the distribution. Red Hat/Fedora-based distributions already use SELinux in place of AppArmor, so using snap on those systems can't have full security capabilities (making SELinux and snap work together would be non-trivial, and I don't think anyone is motivated to do it).
Flatpak uses other mechanisms for limiting the access that applications have, so does not rely on either AppArmor or SELinux being on the host system.
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In practice, I've found Guix, Nix, and Docker to be most useful solutions to missing/outdated apps, though these are more complicated than Snap, Flatpak, or AppImage.