The thing with AppImages is: it requires FUSE2 which doesn’t really get packaged/included by default anymore in a lot of places and the recommendation is “build on the most old and crusty distro you want to support” which just sounds like a nightmare in multiple ways :)
And with snaps the sandboxing only really works on Ubuntu and nowhere else last time I looked into it (then there is also the entire problem if you want to host your own repository/“storefront”).
So really the only universal sandboxing method that effectivly makes sense is Flatpak.