Although I mostly use native software, I find AppImages useful for testing beta software, since they’re one file and easy to try out.
For example: I’ve been using it with the Krita 5.2 beta and I have also used it before for Godot betas.
I use Flatpak when the native package doesn’t work properly or isn’t updated at the rate I’d like, although there are cases where I will use it for other reasons, like sandboxing when I don’t want an app to have access to everything.
I have never used snaps.
From my experience, most of the things I’d like to delete after uninstalling are in
~/.var/app/(App ID)/
.