My favorite aspect to snaps/flatpaks/appimage is for trying out new software without invading my system with tons of misc packages. (aka, easy cleanup).
My second favorite aspect of these is that the software is typically THE most up-to-date software available.
It doesn’t bother me if the theme in the flat-snap-image isn’t the same as the rest of my system, like it wouldn’t bother me the color of the car I’m test-driving.
Looking through my system, I’ve noticed that Steam, LibreOffice, OnlyOffice, and FreeCad are the flatpak editions.
Its a great technology and I’m glad its here. I like that snaps can be either CLI or GUI, as there are a lot of CLI programs that would definitely benefit from being containerized and the most up-to-date versions.
How are you liking NixOS? I’ve heard that its the Arch-of-containers.
On my Debian Bookworm system with Gnome I have some flatpaks. They are Liferea, Skype and Chromium (both because of work). I am finally using Firefox as a flatpak because it is more up-to-date than Debian’s ESR version. Then I also have Kodi, Telegram and KeePassXC (this one should be the newest for security reasons even though I would trust Debian to backport patches to its own version). All the rest is still from the repos.
I am not really the biggest fan of the universal packages, but they are here and are not going away, so why should I not use them? Apart from that, what I really like is that now apart from using the regular repos, I only have to add some flatpaks to my system instead of trying to hunt .deb’s, external repos or random packages from the internet.
I am bothered that e.g. KeePassXC looks horrible, but it works and like @jastombaugh already said, they are really up-to-date. That makes my beloved Debian just more usable than before if you are on stable.
I use Ubuntu and containerized snaps. Almost daily I use Ubuntu 16.04 ESM in a Virtualbox VM exclusively for banking, no browsing the www, no email and no (a)social media. I have installed the needed applications using the latest stable snaps of Firefox and LibreOffice (Calc). Most of the time the VM is not loaded. The VM is encrypted by Virtualbox and its firewall is closed for inbound traffic.
That is the advantage of Ubuntu’s 10 years support, while the newest versions of the apps are available through the snaps. Try that with Fedora 24 and flatpaks