What's your favourite OS that does not use systemd?

Yeah, it doesn't actually make much of a difference:

Fundamentally the idea of having a separate admin account, which is completely protected, and a user account where everything can mingle together and see everything else, is a 1960s security model. It was originally created for a world where the owner of the computer and the user of the computer were two different people. In that world the user provides all the software that they want to run in their account (they probably wrote it) and the OS's job is to protect the admin account from users and the users from each other.

Fast forward to the present day and this security model is completely mismatched with the reality of a personal computer. The internet exists, the user and owner are the same person, and they're probably not writing all their software themselves. A piece of malicious or compromised software can encrypt every file in your user folder, steal your browser history, your saved passwords, and (on xwindows) record your keystrokes and make your screen display anything it wants, all without privilege escalation. But you can rest assured knowing that the user account can't violate any timeshare limits that the root account placed on it.

The one thing you could argue is that a separate admin account makes it easier to detect and fix a compromised user account, but:

1. Most people are not in the habit of regularly logging into their root account and examining all the processes that are running in their user account. In fact many distributions do not even have a separate root account.
2. If you do think your computer has been compromised the sensible thing is to wipe the disk and restore from backup. It just doesn't make any sense to fiddle around trying to figure out just how compromised you are and trying to reverse the process in a running system.
3. If you're running xwindows I hope you never install updates or type your password for any other reason while some malicious software is running, since, as previously stated, anything running under your account can record your keystrokes. In that case your admin account is compromised anyway without having to use any privilege escalation exploits. Can you see how all this stuff was built with the assumption that the user and owner are two separate people with two separate passwords?

With Wayland and containerized applications we are slowly moving away from that 1960s security posture, which is something that's long overdo. But currently something like Linux Mint is not really much better off than Haiku, from a pure security model standpoint.

In any case its security model is not the interesting thing about Haiku.