Here are some infos I gathered about the different cross-distro packaging systems
Ubuntu Snap
+seems easier to create
-probably does not work on older distributions
-requires an external package to run (snapd), and might take a long time until it is available on every distro
-support for non ubuntu distro is not verified
AppImage
+works on many distros, incuding old ones
+requires no external daemon/program to run an appimage
-requires to build on an old distribution and to test every distro
Flatpak
-requires an external runtime package to run, even if using a KDE distro not easily available on older distros