But I’ve been looking at immutable distros as a more stable alternative.

If "stable" is used in the context of "intended use entails no changes/updates to packages found in the repositories in between 'long supported' point releases"^[1]^, then it's important to note that an 'immutable' distro as such does not exist; or at least is far from mainstream*.

If, however, "stable", instead, is used in the context of "less inclined to cause breakage upon (perhaps more frequent) updates", then indeed; 'immutable' distros can definitely be beneficial. Heck, I would even argue that they are successful at providing more stable experiences. This is actually implied merely by design. And, thankfully, the 'immutable' distros have been able to deliver on this promise.

But, it sounds to me like it’s more adapted for smaller devices and IoT, like the Steam Deck or similar handheld devices.

They, indeed, make a lot of sense for these use cases. However, the use of 'immutable' distros on desktop is also pretty well established. Even if it's currently (relatively) niche.

Have you installed an immutable distro on your PC?

Yup. Over two years ago, I switched cold turkey from Windows to Fedora Silverblue without any prior Linux experience. I've been very happy with it ever since. However, since over a year, I've been on uBlue. These are recommended over Fedora's own images for a multitude of reasons; one of which being better on-boarding.

What distro did you use?

As previously mentioned; Fedora Silverblue. Back then, and even today, Fedora Atomic has been one of the most mature iterations. Other mature 'immutable' distros (i.e. Guix System and NixOS) require a lot more know-how by comparison.

What was your experience like?

I simply don't see myself use traditional distros beyond special use cases. Literally all of my experiences with (semi-)rolling traditional distros^[2]^ (that I have engaged with through dual booting) ended with an unbootable system. By contrast, besides my first week, I can't recall the need to resort to Fedora Atomic's built-in rollback functionality to combat a non-booting system. It's just been such a pleasant experience.

1. Besides those related to security.
2. Which include the likes of Arch, EndeavourOS and Nobara.