As far as use cases where immutable distros would be more or less convenient, it appears to me that due to the security and reproducibility factors, immutable distros are better for server or enterprise environments where updates need to be rolled out quickly and smoothly, which are most actual Linux systems out there and where Linux companies make their money (hence why they seem like the hot new thing right now), and for desktops where people are primarily concerned with not wanting to "break" anything. But a classical distro, IMO, would be better for folks like me who do want the control to change or customize things down to the core parts of the OS. As far as I'm aware, you don't have much freedom to tinker with an immutable distro until it's wiped away with the next update.