I agree, if you have a specific dev flow that is compatible with the immutable OS approach, then these can be wonderful dev machines, but personally I don't want to change my workflow to fit the OS, I prefer the OS to fit my workflow.
At some point I I'm pretty confident that I will switch to an immutable version of Fedora and relearn my workflow in a distro box like world as I do see some real benefits to doing so, but I'm not in a hurry