Appimage for me ticks all the boxes for cross distro package as its very portable, simple to run, what are devs trying to do when creating snaps and flatpack?
openSUSE's Richard Brown has given multiple talks over the years comparing these three. I'd suggest anyone to look at those for a great rundown on how these universal package managers compare to one another. His most recent talk can be found here; in which he actually does some kind of recap as well.