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

There are plenty of desktop focused immutable Linux distros. With Fedora Sikverblue/Kinoite probably being the most prominent one, but there are also Vanilla OS, the ublue distros and the one I'm personally using, (openSUSE) Aeon. NixOS technically counts too I think, but that one has it's whole own philosophy/structure that extends way beyond just being immutable

What were the pros and cons according to you?

Pros: increased stability/less risk of breakage, sepaeation of base system/apps that will be more intuitive to many non-Linux users, (Flatpak) apps tend to always be the newest version
Cons: still some smaller pain points around app integration, some flatpaks might have some features that don't fully work or you might need to change a permission (this has gotten a lot better already though), less suited for tinkerers