Immutable distros just one-up that by trying to steer the system in a direction where it can work with a readonly rootfs in normal operation, and nudging you to take a snapshot before/after taking the rootfs from readonly to read-write. (openSUSE has you covered there as well, if that's your thing; it's called MicroOS).
Both of those distros use KDE by default, so the value-add of KDE having its own distribution is basically so they can have a "reference implementation" that will always have all the latest and greatest that KDE has to offer, and showcase to the rest of the Linux world, how they envision the integration should be done.
If I were to set up a library computer or a computer for my aging parents, I would choose openSUSE Leap Micro with KDE, as that would put the emphasis on stability instead.
If you already commit all your changes, anyway, what keeps you from using Nix and running one more command (`nixos-rebuild switch`)?
This is a major reason I ended up with https://getaurora.dev. I layer a few things, but it comes with bells and whistles (like NVIDIA drivers, if you need that).
I can't see myself going back to a "normal" distro. I don't want to spend time cosplaying a sysadmin, I have things to do on my computer.