But is the actual architecture of systemd giving simple, securable, reviewable boundaries? This is a codebase that runs in stages of inherent privelege and very low levels in many linux systems.
Yes, systemd is not one piece of software, it’s a collection of software, like GNU. The components aren’t just modules or something, they’re actually different programs altogether.
When people say systemd they really mean systemd-init, which is one piece of software. Not systemd-resolved, which is another.