That's perhaps a bit too open of a question to ask 😅. But I'll give it a try:

I'll assume the following:

• You asked specifically for the 'immutable' distros that are intended to be used on desktop. Which, moving forward will be referred to as 'immutable' desktops.
• You asked me to look at them in a 'vacuum', thus not comparing it to other 'immutable' desktops. Or at least, it shouldn't be the primary focus.

So without further ado:

• Out of the earlier named 'versions', Aeon (GNOME version) is clearly the most polished and the only one I would actually recommend using. Regarding Kalpa (KDE version); just a few months ago its (then) most active maintainer had stated the following:

'I am stating, right now, for those of you that are clamoring for it to be so, or asking when it will be “release ready” that microOS Desktop Plasma, is not, and will not be “release ready” anytime soon.'

This, indeed, is quite worrisome 😅. Unfortunately, Greybeard (Sway version) is arguably even less production ready... So for starters, if you want to use any of openSUSE's 'immutable' desktops, then you should definitely use openSUSE Aeon.

• Regarding the inner-workings of openSUSE's immutable desktops: -though this is merely an oversimplification- one could understand it as openSUSE Tumbleweed's model with some 'extras'. With those extras being:
  • The base system components of the currently running system is snapshot and copied
  • Changes (be it installing/removing packages (natively) or upgrading base system components etc) are applied on the newly copied snapshot atomically; which means it either happens or doesn't. There's no in-between state, even with power outages and whatnot. Thus guaranteeing that a lot of the complexity with updating that would be found on traditional systems is removed. Btw, atomic updates is almost like a basic requirement with how prevalent it is on any distro that's considered 'immutable'.
  • After the changes have been applied successfully, the copy is made read-only.
  • Changes are then supposed to require a (soft-)reboot for them to take effect.

As this model is relatively 'simple' compared to other immutable distros and doesn't seem a radical departure from traditional systems, one might expect a lot of things to 'just continue working'. However, I'm not confident if that's actually the case. Though, I'd love others to chime in and tell us their experiences. This more simple model does come at a 'cost' though; as it stands, this model is not declarative, nor is it reproducible. Which are qualities found on some other 'immutable' distros.

• The implementation of its release cycle, however, is a major win for openSUSE's immutable desktops and probably the best reason for choosing it over the others. For years openSUSE has pioneered what a stable rolling release is supposed to look like with their Tumbleweed. And its immutable desktops continue to benefit of this. So while blendOS, Fedora (on Rawhide) and NixOS (on unstable) technically are other 'immutable' distros with rolling release cycles, one simply can't deny that they're inferior (in the rolling release aspect) compared to openSUSE's immutable desktops.
• On a final note, I've often heard that openSUSE's 'immutable' desktops have more 'sane' defaults compared to some of the others. Things like offering Firefox as a flatpak instead, shipping Distrobox by default or installing flatpaks not system-wide but per user etc. These might seem like little nitpicks, and arguably others might simply not agree with these choices. However, I agree that generally-speaking most users should prefer these defaults.

Please let me know in case you were expecting a different type of answer!