You could also give an unprivileged user access to run VMs, that has its own set of security tradeoffs, but VM technology is generally considered more isolated

You could also give an unprivileged user access to run VMs, that has its own set of security tradeoffs, but VM technology is generally considered more isolated and mature. Unprivileged user namespaces give an unprivileged user the opportunity to use syscalls like chroot, mount, etc. which they historically couldn't use. Container managers like Docker and Podman drop those "capabilities" after creating the container, so an application running inside a container isn't that unsafe (probably, at least everybody is doing it nowadays for production servers). I think a more problematic scenario is that someone who gets hold of an unprivileged user can then create an user namespace without dropping those capabilities and then use a local root exploit based on those syscalls you normally weren't able to use.