Everything is relative of course but it's a consensus view in OS literature that the Unix family of operating systems doesn't have strong security. There's a huge TCB in which bugs lead to vulnerabilities with high probability, as we see all the time.

This is why eg cloud providers don't rely on the OS to isolate customers from each other.

Since the reneissance of virtualization, many security focused systems have built on that, like Qubes OS, seL4-based virtualized systems, etc.