Root vs non-root is, in a nutshell, a tangled mess with millions of opportunities for "oops, didn't think of that" situations: https://www.cve.org/downloads
Immutable keeps the legacy system confituration basically unchanged, but also restores it "from ROM" every time you power on. (not real ROM, but effectively.) In an immutable system, the attack surface is dramatically reduced, and much more diverse among systems than with a mutable OS.