I’ve actually ran into some of those problems. If you run sudo su --login someuser, it’s still part of your user’s process group and session. With run0 that would actually give you a shell equivalent to as if you logged in locally, and manage user units, all the PAM modules.
systemd-run can do a lot of stuff, basically anything you can possibly do in a systemd unit, which is basically every property you can set on a process. Processor affinity, memory limits, cgroups, capabilities, NUMA node binding, namespaces, everything.
I’m not sure I would adopt run0 as my goto since if D-Bus is hosed you’re really locked out and stuck. But it’s got its uses, and it’s just a symlink, it’s basically free so its existence is kBs of bloat at most. There’s always good ol su when you’re really stuck.