So...

Concept of OSTree or image-based

In theory "immutable distros" are safer to use. Not easier, but setting up stuff is less hard than fixing a system that doesnt boot or upgrade.

I am only focussing on Fedora Atomic desktops, which use OSTree (which is a version control system like git, but for binaries) and in the future/currently in parallel bootable OCI containers.

Both technologies have the same purpose, that your system is an exact bit-by-bit clone of the upstream system.

Layering

Now the system needs to have support for modding, doesnt it? Android doesnt, ChromeOS doesnt, I think SteamOS also doesnt? But this is Desktop Linux!

While many distros use flawed and incomplete concepts, lacking an "escape path" (reset) back to normal (100% upstream with no changes) (for example OpenSUSE microOS, VanillaOS etc), all such distros allow you to change the system.

The disadvantage of image-based is, that you always base of the unchanged image and then add your changes. On every update, you pull down the changes, open that thing up, throw in your changes, pack it again. This takes time and wouldnt be sustainable for example when using a phone.

So you kinda need custom images like uBlue. The advantage here is, that all changes are done on a single system and all clients just clone that. Fedora for exmample has notorious issues with an understaffed rpmfusion team and problems in coordination, so you might get sync issues and a critical security update doesnt work because of a random other package conflict.

or you might get a regression, uBlue could centrally roll that back.

Apps

Tbh the biggest issue is with edge cases of Flatpaks, like portals.

I just now needed to create a signature containing an image in thunderbird. The solution is to copy that image to the internal ~/.var/app/org.mozilla.thunderbird/ container and paste the exact file path there, as portals are broken after app restart.

Then adding an HTML as signature, it needs to be saved in the same folder and also linked exactly.

These edge cases are issues. Let alone missing hardware key support, no filesystem sandboxing in Firefox Flatpak (and uBlue and Fedora people think that is fine) or outdated target systems, because Flatpak needs to work on Debian 11 e.g.

There are also apps on Flathub that are broken, like QGis, or missing apps like RStudio, both known FOSS alternatives to stuff that people really use, and I couldnt even run those without Distrobox, which is also not preinstalled on Fedora Atomic Desktops, and toolbx lacks basic features like separated homedirs.

Yup, it is a rough field. But the stability is worth it. Also, official Flatpaks are great.