I was wondering the same thing. Distrobox mounts $HOME by default, but it’s trivial to give a distrobox its own $HOME.
Can you explain the advantages of systemd-nspawn containers versus podman/docker? I’m not familiar with systemd-nspawn, but I’m a regular user of distrobox and podman.
Nspawn is a full system container engine with its own systemd environment, which makes it closer to LXC/Incus than Podman/Docker. You can do the single process thing in system containers or use a full init system in Docker/Podman, but the experience won't be as smooth.