AppImage is a universal software package format which is self contained with all of the necessary dependencies and libraries in a compressed form. Software in an AppImage is portable and self contained. AppImage can also be described as a software disk image. Runtimes are not isolated, and you are at risk of whatever trojan could be impregnated into the AppImage.
Some say Snaps, Flatpack and Appimage are all ways to dumb down the linux user, are inefficient, and create security holes in the system.
There's no fragmentation with Snaps/Flatpak/Appimage - their installs are completely separate from the main OS.